i'd follow tando's first bit of advice and get a school of tetras. alternatively, you could look at harlequin rasboras. you could keep 10 neons in a 20g. wouldn't recommend clown loaches...they get far too big for anything less than a 55g (and that's to start, as adults they need a much bigger tank). if you wanted loaches, you could probably get a trio of yoyos, or, better yet, 5 skunk loaches, as they stay relatively small. another good bottom feeder choice for a 20g would be a small school of, say 5, cory cats.
remember to stock up slowly! don't add all your fish at once.