I would stay away from plecos. I only know of one that might stay small enough for a 20 gallon and I have never been able to find it in a store.
How are you cycling your tank, if you don't mind me asking?
I'm not very good at helping to stock a tank without overstocking. I haven't learned all the right ways to do it. I can tell you some semi-aggressive fish I like and maybe that will help give you some ideas as to what you would want in your tank.
I like bettas, but for a 20 gallon, I would probably put him/her with a small group of cory cats. You might be able to have a small school of another small fish, but you would have to stay away from any that have a reputation of nipping fins. Bettas are easily made victims of fin nipping, which causes them a lot of stress, leading potentially to other issues with him/her.
Gouramis are another choice. As far as I know, they all can do fine in a 20 gallon, but I have only owned a dwarf gourami. If you wanted more than one, you would have to look into compatibility, because some species of gouramis will fight like two male bettas while others can be fine together.
Cichlids are another semi-aggressive fish. I have not had any, though I would like to. I believe most will not work in a 20 gallon, but some might. I think German blue rams stay small. Someone else may correct me on this one.
I hope this helps give you ideas for your tank!!