Hey big, I've finished Guitar Fundamental 1 & 2 with Lisa, Anders and Chris, and I'm now working on the Advanced courses. I don't remember any of them being chord-nazis, although Lisa is IMO the most "by the book" GF instructor, so I don't doubt what you say.

In the very beginning of the Advanced Rock 1 course, Anders teaches students the "rock version" of a bunch of open and barre chords, including A major which he plays entirely with the pad of his index finger (mini-barre on 3 strings). So, there's not really a "right" way to play that chord. Some fingerings lend themselves more or less to chord transition but before you worry about that you need to do whatever you have to do in order to consistently and quickly play each chord cleanly.

The method that works for me the best is exactly what you describe, that is, mini-barre the D and G strings with my index finger while fretting the B string with my middle finger. Alternatively, I sometimes play it with middle and ring fingers instead. Either way, I'm able to nail an A major easily, cleanly, quickly and consistently. I'm much less proficient when using 3 fingers on 3 strings, and simply can't do it properly at all with just one finger. Just wait until you start playing A-shaped barre chords up the neck, in those tiny frets. It's all but impossible for me.

Just think of all the guitarists out there who have lost the use of one or more fingers due to amputation, nerve damage, arthritis, etc. yet still play with great skill. If you think you have big fingers you should find a video of Israel Kamakawiwo'ole doing his thing. He's the beloved 700 lb. Hawaiian ukulele player who performed the "Somewhere Over the Rainbow/It's a Wonderful World" mash-up that you've undoubtedly heard on the radio. He was extremely adept at playing the ukulele yet he rarely played chords in their tradional forms because of his bratwurst-sized fingers.

Nicolai