could try rams... rams stay small and are interesting fish.. not sure if the betta will like them and vice versa but you could try it...
rummy nose arent the most hardy fish.. i donno i just dont like them really
i think they miiight nip the bettas fins.. they swim fast and would freak the betta out i would think..
bala sharks do grow large and will eventually outgrow your 20g.. they are also schooling fish and dont feel comfortable being alone really..
gouramis are interesting but they dont get along well with bettas..
could always put in some good ol tetras i guess ... like maybe silver tip tetras
i donno
cories are more of a clean up fish.. they will eat excess food on the bottom of the tank