For a mature tank you can look into Hillstream Loaches. They have very specific needs including temperatures. Reticulated like a little cooler water.
Cross check all stocking options to make sure their tank, water parameters, temp and tank mates all work together.
Rainbows can cross breed so when you decide, make sure you get some, if you select different colors just make sure they are different genus like Boeseman's Rainbowfish (Melanotaenia boesemani) or Millennium Rainbowfish (Glossolepis pseudoincisus) or Madagascar Rainbow (Bedotia geayi) Or different subspecies, so you don't get cross breeds.