I'm a classical guitar student but I always loved the piano. Though I never owned one, most of my musical ideas come to me as harmonies on the keyboard so I often have to translate that into something playable on the guitar or the way around (when I compose with music software).
Which would be the main guidelines one should follow when
I think I see two slightly different questions here. It seems like you are looking for guidelines for writing parts on piano to play on guitar and also how to arrange a piano part for guitar. I will assume you are playing guitar in the standard tuning.
The largest difficulty you can encounter will come from the density of the chord voicings. On a piano, you can have 10 voices, on a guitar you can only have 6 (this is with each finger triggering only one note per on the piano but there are plenty of ways to play more than that). Beyond that the piano can accommodate clusters of notes, which are several notes within a small range, and a guitar has no such capacity. Even a simple triad in root position can be uncomfortable to play on guitar. So if you are trying to write music on the piano to play on the guitar, you should make sure that the chord voicings are playable, paying attention to the density of voicings. Triads are most easily played in 2nd inversion (5th in the bass) on guitar (and first inversion is more preferable than root position). It is very easy to write a part on piano that is impossible to play on guitar.
If you are trying to arrange a piano part to play on guitar, then you need to find the most important pieces and hang onto them when it gets translated to guitar. The most obvious thing here is the melody. If there is a melody, or even somewhat melodic motion, that must be preserved. From there you either need a good ear or some good theory. There will be a lot of time when arranging a piano part for guitar that you won't be able to play all the notes. In these circumstances you will have to boil down what is there and attempt to maintain the harmony. First, you can remove any repeated notes within the harmony. Then the least important voice in a harmony is the 5th, so it would be the first to go of the unique voices. You will also want to pay attention to the voice leading. If there is linear motion imbedded in your harmony, you may want to hang on to that.
The idea is that you are trying to find what about the piano part makes it a 'part' in itself. Why is it a part and not just chord changes? It is the melody and the motion within the harmony. You are looking for what makes it specific. One easy rule about how to translate from one instrument to the other: Sing the part and whatever you sing has to be in the arrangement for the other instrument, as it is definitively the part.
Addendum:
When writing piano parts on a guitar you will want to be mindful of the rhythmic nature of the guitar and the use of muted strings. Chords played in fast rhythms that are strummed out don't translate well to piano. One, because the motion is more physically demanding on the piano, and two, because the mechanics of a piano require the key to depress before it can be struck again. Pianists will often use more than one finger on a single key to play faster rhythms.
Muted notes are also hard to emulate on piano. There is no effective way to play a muted note on piano. A lot of guitar parts will include strumming the whole time and muting the strings at times to create a syncopation and playing that note for note on a piano would be impossible. The best you can do is try to substitute staccato stabs on the piano for the muted guitar notes. The rhythmic nature of strumming a guitar can bring a lot of groove to the song/piece and this groove is what is hard to translate to piano note for note. This mostly applies to rock influenced music, including funk/fusion.
Tone is another important thing to consider. On a guitar most notes can be played on more than one string and there are some notes that can be played on 4 or 5, depending on how many frets you have. Each string will have a different tone based on the gauge, material (wrapped or not, nylon/steel/etc., round/flatwound), and how far up or down the neck the note is. The tone will also change based on where and how you pluck the string. Playing close to the bridge will generate a thinner, sharper tone, while playing toward the neck will tend to be a warmer, more round tone. As a Classical Guitarist, you may have noticed a slight difference in tone from finger to finger. Some of this will come from where the finger is plucking the string, as I just mentioned, but the other difference comes from the hardness and width of the fingernail or finger. These variations in tone can add a lot to the overall texture and feel of the piece. The primary means of changing tone for a piano is to play another piano... beyond that, you can change the tone most dramatically by playing louder and softer. Louder notes tend to be brighter and a softer attack will give a softer tone, which is also true for guitar. The problem here is that this changes the volume of the piano. If you are using an electric piano, you can adjust the volume but if there is a lot of dynamic variation, then it makes for a difficult performance. The endless possibilities of tone that you can get from a guitar, even without amps and effects, can make translating the feel of the guitar to the piano difficult.
If you are arranging guitar parts to play on the piano, then what I mentioned previously about getting the melody and finding what makes it a 'part' is the most important thing. Handling things such as the rhythmic nature of the guitar can be managed through arpeggiation or voice exchange. By breaking the notes of the strumming up into two or more voices (left hand/right hand for example) or arpeggiating, faster and/or persistent rhythms become more playable (as well as more pianistic). Depending on the dynamic depth and the amount of timbral variation, you may have to make a lot of sacrifices but you will want to find the middle ground. Be a little loud for the tone if it is just a short section but don't play loud for tone for the whole piece, as this will affect how the rest of the players will have to play (unless it is a solo piece).
Ultimately when you are arranging a piece to be played on another instrument you want to capture its essence and you want to make it sound like that instrument is supposed to be playing it, such as arpeggiating chords being more pianistic than pounding out each note continuously. Similarly, on a guitar strumming out the chords that are arpeggiated in a piano part may sound more like an authentic guitar part, depending on the genre.
