You said depending on fish. I posted my stock above. Will Bolivian rams work? (If so, do I need 1? 2? A pair?)
If all you have are the neons and panda cories, then a male Bolivian Ram should work. The rainbowfish add an uncertainty for me. A 29g is not a large space and the Bolivian might or might not pester them, I've no idea. The Ram will eat shrimp if he can.
A "pair" or rams means a male and female that have selected each other from the group and bond. Even this is not a guarantee of harmony, but it is usually.