YoYo Loach's
Dojo Loachs
Zipper Loachs
Bolivan ram's (id only add 1 of any ram or Dwarf cichlid species)
German Blue Ram
Pearl Gouramis
3 spot gouarmi
Opalaine gourami
Tiger Barbs
Rainbowfish
African Butterfly fish
Siamese algae eater
Otocinculus
Striped Spiny Eel
Rainbow Shark
Severem's
First off, you realize you're going to need another 3 pearl danios, yes? Three will get to be nippy little buggers, so they need to be in groups of six or more.
As for the rest...
Yoyo loach-Will get too big for your tank (6-10") and needs to be in groups of 3 or more
Dojo loach-Minimum tank size for dojo loaches is 30 gallons and they are true coldwater fish that do not like temps above 75F.
Zipper loach-This one would actually be okay for your tank, just make sure to get at least 3.
Bolivian or German blue ram-One of these would be fine in your tank size.
Pearl gourami-You could have one of these (but no other gouramis).
3 spot gourami-You could have one of these in your tank (but no other gouramis).
Opaline gourami-Same as the 3 spot gourami (again, but no other gouramis).
Tiger barbs-Absolutely not. They need to be in big schools (7+) and are notoriously aggressive/nippy. They get to be 3" and are too active for a 20gal.
Rainbowfish-Depends on which species. Dwarf neons and threadfins would be okay. I'm not sure about the others. Please specify which species you're referring to.
African butterfly fish-One of these would be fine, but with only very PEACEFUL tankmates.
Siamese algae eater-True siamese algae eaters get quite large (5"), so you could only have one. Take a look at this article:
Will the Real SAE Please Swim Forward? Epalzeorhynchus and Crossocheilus et al.; or Minnows as Biological Algae Controls; In Search of the Elusive SAE's (Siamese Algae Eaters)
Otocinclus-I wouldn't recommend them until your tank is AT LEAST 6 months old, your water parameters are PERFECT and you have a good, steady supply of algae. If all those criteria aren't met, you'll lose them pretty quickly. They also like to be in groups of 5+.
Spotted spiny eel-They need at least a 35-40gal tank (absolute minimum), so that's a no.
Rainbow shark-The general recommendation for rainbow sharks is a minimum 55gals because they are territorial (as they are bottom-dwellers, they wouldn't work with rams). I have my albino rainbow shark in my 29gal and he's fine. I WOULD NOT recommend putting one in anything smaller than 29gal however, so this one is out too (they get to be 6" long).
Severums-They get to be 12" long and are VERY aggressive (typical cichlid), so absolutely not.