Welcome to AA
Glad you decided to not keep the angels, I'm no expert but afraid you could have issues in a small tank.
Rainbows are wonderful fish, I have neon dwarfs in my 75 and preparing to get Australians. Minimum tank size for the Australians is 30 gallon, but they do require a 30" long tank for more swimming room.
Your tank would be at the bare minimum and kinda crowded. Here is a link for both the dwarfs and Australians, I might consider the dwarfs in your situation.