Welcome, a 20h would be the bare minimum for a breeding angel pair. I wouldn't add any tankmates or keep them in there long term, either.
As a long term home I'd look at a 29+ for one pair. The fry will need a growout tank also, 20g is good for a few weeks but large spawns will need more tankspace. A good breeding pair will have fry every week to two weeks so plan accordingly. Good luck, if you have any questions just let me know.