New 20 Gallon Tank What Fish Can I Have?

[Dunkal]

You can get Dwarf cilchids which only grow to 3 inches.

 

[rabbut]

There will be mixing problems with the Betta and the angel, so one or other needs to be cut. Your also quite haevily stocked with that many fish in your tank. Angels are large fish, and need at least a 30g tall (18inch+) space-whise, so I'd personaly cut the angel. All other choices are fine
Make shure you get the bristle nose at a reasonable size though, as red fined shark have deceptively large mouths. If they eat a plec, both fish will die, as the sharp fins on the plec will slice the internal organs of the predator fish through digestion, killing the predator
HTH
rabbut

 

[Amunet]

Or you could maybe put one male betta in with say 6 female bettas and one angel and that's it??

Bad bad BAD idea. You should NEVER put a male betta in with females.
Sure you'll hear people saying that they have this, but more times than not, your bettas will fight.
Even in a sorority tank of all female bettas, you still have to have plenty of hiding spots so they can retreat if they're getting picked on too much... and even then there are still some bettas that just don't get along with anything and need to be kept alone.

A female betta might be ok for a community tank. They USUALLY less aggressive than males and come in alot of gorgeous colors. It all depends on the fish though and you could still end up with a very aggressive female.