IMO, your tank is too small for the Congos or Rainbows. Cardinals are the right size, but I want to recommend looking at Diamond Neon Tetras, if they are available in your area. They are stunning. The red is brighter than the Cardinal, it's more like the Blood Fin Tetra. The portion that has no color, like a regular Neon, is the diamond part and sparkles and reflects just like little diamonds, thus the name. You can see the sparkle from the ambient room light available, where Cardinals would be dark and hard to see.
It's a harder to find fish, just remember...it's a Diamond Neon Tetra, not a Diamond Tetra, that one is completely different. It's well worth the look for your size tank.
After 50 years I do a lot of things right, but still get things wrong