Cheap ricordea or zoas

[BlueWorldAquatic]

you can have 10+ fish in that tank, just depends on what you get.

small gobies, clown gobies, etc are all fine.

if you have a clownfish, consider an anemone, as it will give him a home, and he won't travel around the tank much.

As I said before, mandarins are tough, most tanks under 20gal can't handle them unless you get a conditioned one. Pods will be depleted very fast, and he may starve.

If you want corals, angels are not a good choice even pygmy, they all can get a taste for corals.

I would not recommend chromis, for their schooling instincts, they require more space.

Honestly, there are only a few LFS that would recommend that many fish in a 20gal tank.