Hi Emily, Welcome!
I have a small school of harlequins (10) that I keep in a wide two foot tank. (they will soon get a 3 foot one, yay!) Then I'll get more of them...
I've had them only a few months, but they are really great fish. However, I did have a few die soon after getting them, so I wouldn't add them first, wait until your tank is mature. I think I hurried the introductions a bit.
Mine are currently in with a pair of kribensis and a male betta (Siamese fighter) with no trouble!
Cheers, Hellen