i dont know much about shrimp as i dont want any yet tbh, i have 6 loaches, botia kubotai (polkadot loach) that i absolutely LOVE in my tank at the moment and id recommend them anyday!!! One thing you cant have with them though is snails....as they are a favourite food! I also think these will be much too big for a 20 gallon unfortunately.....but definitely something to think about for when you get your 55 gallon
coroes though i think you could put a decent group in your tank, BUT these require sand substrate really, so that would be a switch, but most people i think would tell you its worth the hassle as they are a great fish.
rasboras on the whole are quite small so you need to consider this when matching them with future fish, though again shouldnt be too much of an issue in a smaller tank- though i dont know how they fit with your betta.
there are quite a few plecos that are limited to around 5 inches that might be a possibility for you- bristlenose pleco, queen arabesque (which is what im getting later!), clown pleco, golden nugget (maybe 7 inches for that one off top of head) amongst quite a few others.
neons i just plain dont like because everyone has them. lol. But theres SO many tetras to look between. guppies, again, i just dont like them really. haha.
NOOOO idea about frogs either! lol