I used to keep a fairly wide variety of fish. No matter what you use, a tank is always going to need regular cleaning, but a good filter will certainly help. My favorite tank included a small school of neons, two khuli loaches, two clown loaches, a pleco, a few sword tails, and occasionally a couple rainbow guppies. Sometimes I had a couple of those little catfish type critters, but that was only if we felt like getting one. With a 20 gallon tank, you can safely put a fairly big group of smaller fish, as long as they aren't going to eat each other.

ALL of those fish proved pretty resilient for me, as, I hate to say it, but we wanted to get rid of the tank and no one wanted the fish, so we didn't really do a whole lot and none of them died. (Not sure that we had rainbow guppies at the time though, they don't seem like the surviving type.)