Woop, 20g tank gives you quite a choice of options!
As T_L_M says, you could have Corys on the bottom, and a shoaler for the top, and perhaps a pair of something as a centerpeice.
How about;
6-8 Corys (Panda's are lovely, i have 10, but they a real pain, you have to be VERY attentive to maintenance, very fussy, prone to everything it seems. You need to keep a very clean tank floor to stop them getting mouth fungus.
10-15 Tetra
And a centrepeice. Was going to suggest a pair of honey grourami or somthing of the similar, but they would seriously upset the Betta. (Betta's take a severe disliking to "conspecifics" or, "fish that look like them". Gourami's and Bettas dont seem to get on well for the most part, but some have reported sucesses.)
As above, opersonally i would re-house the Betta. If you can find a cheap second hand small tank (like fluval Chi, or something similar) this would be ideal and would free up a lot of other options for the tank)
EDIT: Be careful also if you do decide to go with something similar to the above as lots of fish often picked as "centrepeice" fish will eat Shrimp....