Effects of downtuning...

Hmm, bigger strings shouldn't be causing fret buzz. If anything tighter string tension usually helps avoid fret buzz.

I think the reasoning is probably that for a given pitch, bigger strings will have higher tension and therefore less buzz. Same reason some people use longer scales for low tunings.
 
I use 10-52 for both standard and 1/2 step down. So, I guess I'm a regular gauge and heavy string guy? ha! A lot really depends on the tone that you're going for, what type of pickups you're using, what your amp rig is like, bla-bla-bla. Heavier strings will tend to emphasize bass frequencies, lighter strings tend to emphasize treble frequencies. I've heard guys play with heavy gauges and sound too trebly. I've heard guys play with 9-42s and sound too bassy. Of course, we can also factor in string composition which also makes a considerable difference. Seriously, this is like arguing length vs. girth.
 
I'm currently using 010-046's and a 056 B in my 27" scale 7-string. They feel pretty good. Earlier it was 010-052's and a 068 B, but the lower strings were kind of muddy with those, so I moved to thinner strings. I don't like the feel as much, but they sound more open and clear, which is definately nice. A 060 B-string would probably be the best, but Elixir doesn't make those, so I'm stuck with 056's.
 
I'm thinking of tuning my Flying V to C / Drop A#, 12-56 would be pretty much the standard choice for C, but is the 56 big enough for dropped A#?