My lents are in a 210 gallon with four tankmates and I definitely wouldn't go smaller. I like the extra 6" width of my 210 over my 150 (both are 6' tanks, but the 210 is 24" wide whereas my 150 is only 18" wide).
I was having some aggression problems, but now that I have them set up on one end of the tank with a bunch of driftwood to call their home, they mostly leave everyone else alone. I see them lip-locking and chasing a few fish occasionally, but there's been no serious damage.
I think both of mine are boys.