6 good sized corys on their own would fully stock that tank.
In my 20 gal, my Dwarf gourami is the "pretty fish" main attraction!
You could have 3 corys and some tetras. I like neons personally, and there is a beautiful golden tetra that's very nice. It's best to stay away from certain laterally compressed species, like Serpaes and Skirts, since they do nip fins.
If you want rams, you'll have to forget most of the other fish, except for maybe some tetras. Cichlids are highly territorial and aggressive when spawning and need lots of room.
An angelfish grows too big and may very well fight with the gourami since both are territorial.