Is a 261 big enough? Depends on the size of the stumps in your area. I would say no, but I don't know your market. I would think you want at minimum a 460, preferably a 660.
I would say no, a heavy machine would not have less vibration. A heavy machine would have more power, so you'll hit the stump harder. It just wont vibrate as long per stump.
Is a big heavy machine a benefit, again it depends on your market. If you are working in yards that are quite soft, you may be dealing with ruts in the grass all the time. Problem? Again, depends on the market. A high end super fussy market - yes.
Again, I would suggest you try it out. Maybe rent one for a week, line up a few jobs - even for a loss and see how you do. There are definitiely tricks to make yourself more efficient (aka profitable).
Also don't forget rakes and blowers, even if you don't do cleanup, you should leave the property neat.