A 20 high is borderline for kuhli loaches. I most certainly wouldn't keep them in anything smaller. A 20 high is as small as I would attempt to keep a betta with other fish. Whether or not it will work will be dependent on the betta. The kuhlis aren't going to cause any trouble, that is for sure. I can't imagine that the betta would have a problem with the loaches either. As for the rasboras, I've not kept them but based on others accounts of them, I would expect them to be a high percentage choice. You really won't know until you try, but it's definitely something that I would try and expect to work.
125 - BGK, chanchito cichlid, pictus cats, silver dollars, palmas bichir
125 - cichlids (severums, bolivian rams, chocolate), rainbows ( turquoise, red), loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, tiretrack eel, pearl gouramis, betta
90 - Congo tetras, african knife, upside down cats, spotted ctenopoma, kribensis, delhezzi bichir
2.5 - betta