Mine are great. I have 3 in a 3-foot tank. They hang out together, and occasionally give one another chase but nothing serious.
Yes to floating plants, yes to lid.
They will eat anything that is on the surface, including pellets and flake. Mine will eat out of tongs - it took a little while to get them comfortable with it but I recommend this, just because they really won't go down off the surface to eat (like my old hatchets used to). I feed them fish, krill, and blackworms out of the tongs since those things don't float very well.