What are the main differences between using a pick vs. playing guitar with my fingers or even fingerstyle? Will there be any negative effects if I get used to one method and later want to learn a different method?
I think that a well-rounded guitarist should use both. Regarding negative effects, my answer is no, provided that you continue to occasionally go back and practice the other style. For example, if you've been using a pick for a long time, and then decide to play fingerstyle for an extended period of time, don't hesitate to go back to using a pick every once in a while.
I believe that the choice between pick and fingerstyle should be based on what type/style music you want to play. But on the other hand, no matter what style you choose, you should use the method that you feel is more comfortable and better-sounding.
The main differences between picking and fingerstyle: this is a broad question, but put very simply, fingerstyle allows you to more easily play in an arpeggiated style.
Playing with your fingers allows you to play multiple notes on distant strings at precisely the same time and to play cross rhythms with ease.
It is, on the other hand, a lot easier to learn to use a plectrum and it is especially much easier to learn to play fast using plectrums.
There are no negative effects with learning to use one. They are different techniques that are mutually exclusive.
From my personal experience, I've found that some pieces are easier to play with a plectrum, especially if they use techniques such as sweep-picking, while others are very difficult, if not impossible to play unless you play with your fingers or finger picks - for example, Recuerdos de la Alhambra requires tremolo picking the top note while playing various notes in the bass, which, if anyone manages to play accurately with a pick, I will immediately be a fan of.
Some of this answer will address your original question before all the edits.
Learning to play the guitar is very rewarding, but the rewards come slowly in the beginning and require dedication, persistence, and commitment (and pain tolerance).
One thing that has become abundantly clear to me after many years of playing guitar, is that no matter how much I practice, I can always get better. And I can always learn new techniques. As a guitarist, no matter how well you master your instrument, you will never get to a point where you cannot improve. There is no plateau - you just keep climbing higher and higher. That's one thing I love about it, I never stop growing.
In the beginning, the improvements are hard fought. Because when you first begin to learn to play the guitar, you can't even get your fingers to contort into the strange positions required to form a G chord or a C chord or an F chord. So just finally playing your first chord is a major accomplishment.
And learning to form chords is a big part of learning to play guitar. In fact let me go farther and say it is the MOST important part! Whether you play with a regular pick (aka plectrum) or a thumb pick and finger picks, or with your bare fingers, you won't go very far if you can't form chords with your fretting hand!
Another thing to be aware of is that often you will play the guitar as a "rhythm instrument" especially when you are at the beginning stages of your journey. And by that I mean that in addition to strumming chords and playing notes to go along with whatever song you are playing, you will need to keep the rhythm or beat of the song in a consistent and steady rhythm -either with strumming or the notes you emphasize while picking (either with pick or fingers).
Most beginners find the simple strumming and picking patterns easier to learn before advancing to more complicated strumming or picking patterns. I have observed that timing and rhythm are one of the more challenging aspects for many beginners. It takes practice to develop a coordinated sense of rhythm.
And when learning to play new chords and notes while maintaining the proper rhythm is a battle in an of itself - whether strumming or picking with fingers or a pick.
In the beginning there is so much to learn that are not affected by your choice to play with fingers or pick. As you progress with the guitar, you will want to learn many different picking hand techniques. None are mutually exclusive of the others and most accomplished guitarist employ many different methods, techniques and styles.
There are multiple ways to get those strings to vibrate. You can play with a light pick, a medium pick, a heavy pick, a thumb pick and bare fingers, a thumb pick and finger picks or long fingernails on your picking hand. You can strum with you thumb, the back of your fingernails, or your fingertips. You can pluck the strings with your finger tips (one at a time or up to 5 at a time if you have 5 fingers), you can play finger style with anything from simple repeating patterns which alternate between bass and treble strings to complex patterns. You can play with a combination of a regular plectrum (pick) and finger tips, finger nails, or the back of your finger nails with a percussive strike to keep a drum beat going.
After you master chord formation, chord changes, and basic rhythm, then you can begin to explore different picking/strumming (right hand - if you are right handed) techniques. This can be a combination of strumming chords with a pick, and learning to pick out individual notes with a pick and learning some simple (to start with) fingerstyle patterns that you can play with a thumbpick and fingerpicks or long fingernails.
Learn to form some of the basic chords and learn the chord changes required to play some songs you like. Learn a simple strumming pattern like a down down down down in 4/4 time. Then learn more complex strumming patterns starting with down up down up and then down - down up - down - down up.
This is not a prescription - I simply want to point out that there is much to learn before you even begin to worry about your strumming/picking hand technique. And if you never get to the point where you can play a simple song, you won't even begin experimenting with picks versed fingers verses fingerstyle verses clawhammer, verses hybrid picking, verses etc.
Bottom line (TLDR) - the answer to your question is use whatever method is most comfortable for you to play the correct strings with the correct rhythm and timing while you learn the basics of chord formation, chord changes and simple rhythm. After you get past those simple basics, you can start learning different methods of picking, strumming or plucking the strings. And I promise, learning one way will not impair or hinder your ability to incorporate new techniques into your playing style as you advance.
Good luck with your goal of learning to play guitar. And if you stick with it - you will soon see that you are getting better every day!
It is important to note the differences in attack, as well, that are available.
Though it is a generalization that master players mitigate through careful technique, use of a plectrum usually introduces an attack with a different sound profile than the attack one hears from finger picking.
Of course there is is continuous spectrum of softness to harshness available via either technique, but the qualities of attack are pretty easily distinguished.
Any classical player serves as a good example of finger picking attacks. I would add Derek Trucks and Mark Knopfler on the electric side.
There are any number of flat pickers to choose from, either acoustic or electric, but I believe listening to some samples will bear me out, and give you the idea.
Using simple examples, just as Michal has never seen 'Recuerdos de la Alhambra' performed with a plectrum, I have never seen 'Tumeni notes' performed finger style. I can't say either is impossible but then, neither is moving Scafell Pike with a soup spoon - how much time do you have to work on it? There are no negative effects of trying either provided you put the hours in.
I would advise you to first master one type of picking technique before you go on to another. It takes years and many hundreds of hours of practice to truly master the picking technique so if you are constantly changing between the two you risk becoming a jack of all trades and a master of none.
Main difference is the amount of notes you can play together and space between the strings you can play them together.