we keep all of our lions and other scorps at SG 1.023 at 76*F - 77*F.
i think you could keep a trio of dwarf lions nicely in a 55 gal as long as your filtration is up to snuff. feed them 3x a week (we feed M-W-F), and vary the food you offer them.
IIRC, rusty_spoonz kept a fuzzy, a zebra, and a fu manchu together. his "fu" was weaned onto dead foods, and had no trouble with competition.
we'll be keeping a fuzzy, a D. barberi, and a fu together in a 60 gal + sump once they all reach the right size. our "fu" is weaned to dead food as well.