Those livebearers have a pretty high bioload already, check out aquadvisor.com to see what kind of stocking you already have and you can experiment with adding in different schools to see what the stocking would be. Cherry barbs, smaller varieties of tetras, and rasboras are all good options that are usually readily available at pet stores.
White clouds are a coldwater species and not a good idea
I love neon tetras, glow light tetras, and ghost catfish
It's a bit of a misconception about them. They are a hardy fish that adapt well to warmer waters. A tank that's kept at 76 degrees will suit them fine. I think people who keep their tanks at 78+ may shorten their life span.