Abram, the 50/1.2 and 58/1.2 are pretty big and heavy on the compact FM3A but manageable if you must.
What I find much more an issue then the size and weight of the lens is that the little FM3A finder is ok to focus the fast f1.2 lenses but the finder of a (cheap as dirt) Nikon F3 is so much better and easy on the eye (especially when using the high magnification finder and a full matte screen as the H2 screen).
This is why I barely ever use the fast lenses on my FM3A but if I can use the F3 instead.
I find lightweight and small lenses most wonderful on the FM3A - the perfect pairing.
My favorite lens on the FM3A is the 50/2.8 AiS - perfect handling (and a very sharp and contrasty lens to boot).
… not saying the fast glass can't be used on the FM3A ;-)
Cortado on Nikon-in-the-bag-day by
Dirk Steffen, on Flickr