Hi gillyg. I wouldn't get a clown loach as it will outgrow your tank. If you're just looking for bottom feeders/scavengers to clean up left over food, corys are great and so are ghost shrimp or amano shrimp. Amano shrimp will also eat algae.
You need to keep a very close eye on the fish you currently have. Tiger barbs are extremely aggressive fish and really aren't good tankmates for the mollies, gourami or any slow-moving fish. They will harass and nip at them and most likely will end up being the cause of their death. TBs really aren't suitable for your tank either as they need lots of swimming room and should be kept in groups of at least 6 to ease the aggression.