There's absolutely nothing wrong with playing A using the 2-1-3 pattern, especially if you have fat fingers. (like I do) Even if you can fit your fingers in a row...the 1-2-3 pattern....there are good reasons to do it the other way. I have always played A using the 2-1-3 formation.
Consider this:
Look at the chords A, Amaj7, and A7. In fact, try changing between those chords. It makes an interesting transitional change. If you use the 2-1-3 fingering, the entire change is made by moving your pointer finger down one fret and then lifting it to play the G string open.
A........... x-0-2-2-2-0
Amaj7.... x-0-2-1-2-0
A7 ........ x-0-2-0-2-0
When learning to transition smoothly and quickly between chords, it helps to use pivot fingers. If two chords have a common note, there is no reason to move that finger. It acts as a pivot and reference point so you don't have to look at the strings. With that in mind, look at the A - D transition. If you use the 2-1-3 fingering, you can leave your 2nd finger in place on the G string, and pivot on it. the other two fingers move to convert to a D chord. This is a very common change, and using the 2-1-3 fingering makes it a lot easier than the 1-2-3 fingering.
As you progress, you may find yourself using multiple fingerings for the same chords...depending on what chord progression you're using. Nothing is cast in stone. Having said that, it will be to your benefit to have a flesh and blood mentor. There's a lot of great instructional material on the web, but it's no comparison to having somebody give you real-time feedback. Best of luck to you.