I have a question regarding the use of the pinky on smaller frets , 10 onwards . Let's say I am learning a guitar solo, from Metallica Nothing else matters, when they alternate from fourth srting fret 12 to 15, is it necessary for me to have my first finger on 12 and pinky on 15 ? There is more than enough room to just use the third finger, makes my playing a little faster too. Is it really bad practice to do that on small frets ? Mind you I am only beginning so I don't want to pick up bad habits. Thanks a lot !
Some players' fingers are too fat, too short, too long to do what others are happy with. Part of learning a new piece is to decide which fingering is best for you, and it seems like you are well aware of that factor. Try all options, and if the fret space is big enough to accommodate whichever finger, and the stretch isn't making your hand work harder than it needs, do whatever you consider the most comfortable, convenient and effective.
If you're working from a book, or the 'net, and fingering is there for you, it's there for your guidance, not etched in stone. Probably what someone else decided was best - for them. You're not them, so use your own fingering. Django did..!
It is very common to use the 1 and 3 over a minor third (3 fret stretch) up there. In fact it is common to use the third finger on that pattern even below the 10th fret, say at 7 or 5. It all depends on the physiology of your hand and if you can do this comfortably. As for being faster, that is a matter of practice. In time there is no reason why you shouldn't be able to trill with any two fingers at the same speed and strength.
Many Blues and blues rock players go up the minor pentatonic "box pattern" scale exclusively with fingers 1 and 3.