Will these two species get along in a tank
I'm not sure about yours, but my rainbow wad a bully, I had to rehome him
What size tank?
20 tall is waaaaaaaaaaaaaaaaaaa...... wait for it....... aaaaaaaaaaay too small for a rainbow shark, or any kind of shark for that matter. Your dojo loach is not suitable for a tank that size, either. They need friends, besides the lack of space. Edited to add: To answer your question, with a large enough aquarium, I think they could coexist. 20 high is not that aquarium.
Yes. As long as they are in a 5 gallon or more they should be fine
No... absolutely not, and smiles don't make it better. I wouldn't keep either in the absolute minimum of a 55 gallon aquarium..
I know plenty of people that have rainbow sharks in 20 longs. Not everything has to be perfect
a 20 long is insufficient for either fish, and you have a tall. Rehome your current fish and grab some fishpeople that are compatible.