just teh danios, the other fish need diffrent water conditions than the shellies. If you really want some Id suggest you get that other tank you want, and ditch the convicts, as a 20 gallon is not a good setup for them even though some places say it is okay, it isnt. Then in the 20 you have now keep the danios and get either
2 pairs of brevis
or
5-6 multiesfacitus
or
1male and 3 females ocellatus
I believe that either of those should work. I recently did alot of resurch on shellies my self because in march I will be odering some, I am just not 100 percent sure about the ocellatus, still doing some resurch on them. I do know that in a 29 gallon you can keep 4 pairs of brevis, and that has the same foot print as the 20 gallon, so you minght be able to get 4 pairs of brevis.