Well thats just up to you, I kept clown loach from a friend of mine with yoyo inside a 20g which they seem happy atm altho fighting. But being a loach lover I would say get one or two of the queen/zebra should be fine as they r funny to observe. And the gravel doesn't have tp be sand. Sand yes is probably the best but I dont use sand either on my loaches tank. Small round gravel also works wonder for queen/zebra already.