Most tetras need to be in a group of 4 bare bones minimum, with five or more of the same species being much better. Buenos aires tetras are known to be more nippy and with only one, that may make it even more agressive.
For a fish in, I'd have no more than 3 smallish, or mabye 4 if tetras to help give them a little school, in a 29 gal, but others may disagree.
I'm guessing a local chain store sold you this combination? I'd see if I could rehome the tetra for sure. I don't know much about Rainbows, but you already know it wants company, so rehoming it would be best, also. Adding even more fish at this point would not be a good idea. The mollies and platies would be more than enough to start with.
Good luck with it. I have a 29 gallon tank too, and im fishless cycling before I get my first group of tetras. I'm sure you are reading all you can about fish in cycling and will stay on top of the frequent water changes. That's essential to success and the health of your fish.