What species of Rainbowfish? There are several!
What size tank do you have and what species of Gourami? What numbers of Harlequins do you have and what species/number of Tetras?
Dwarf Neon Rainbowfish are peaceful shoaling fish that get only 2" and really need over 20 US gallons to shoal properly. With those mix already you would need a pretty large tank to consider anything any more fish.
