So I don't know a darn thing about plowing snow but I've run b6100s and 7100s with a back blade and a loader, so..
For one thing 60" seems about right, and i would NOT go any narrower because i know a 60" back blade is irritatingly narrow when angled. I'm looking for a bigger back blade.
Second problem though is that without enough weight on the back of the tractor, an angled blade can drag the back of the tractor sideways. I imagine this issue is the same on the front. One thing I can say with what little knowledge I have, is if you pull up on the blade not enough to actually lift it off the ground, but a little less than that, you're putting that much weight on the tractor which will help it go where you point it. Should work even better on a plow than a back blade.
As far as lifting it.. why couldn't a b6100 lift it? My factory loader does 500lb to full height, probably closer to 800lbs off the ground, and the loader runs a whopping 750psi (stock onboard 3pt relief is like 1700 iirc). Seems like if a b6100 couldn't pick up a plow it just means the plow mount wasn't designed right. Does this have something to do with using the 3pt in a very not-ideal way to lift the plow which is 7-8 ft away from it?
If you mount it under the middle of the tractor, the weight the plow is pushing will help pull down the front wheels for 4wd traction. If you take the easy route and just mount it to the front of the frame, you would not be able to 'float' it because the load you're pushing would just try to pick the front wheels up. You could pick up 90% of the weight like i was saying and hold the front end down but if you were on uneven ground it wouldn't follow it. I don't know if people snow plow very much undulating surfaces, though.
Btw, NICE looking tractor! My 3000hr 6100 is pretty ugly!