I think any kind of Angel is a bad idea in a tank that small. Pygmy Angels need 50+ and Dwarfs need 70+. Sorry, but IMO
, putting a fish like an Angel in a small tank is begging for problems, especially with clowns. It's not the typical Tang argument (need lots of swimming space), its more of a territorial thing.
If you are concerned about keeping an additional fish alive, I would keep within the general rules or be ready for more problems.
I wish I had a LFS
that did returns for saltwater. I've been doing aquariums since I was a kid. Probably frequented over 20 LFSs over the last 30+ years and never found one that took returns on saltwater.