The general rule of tetras for schools is 6 a piece. They may school together now (I had black neons and rasboras that did that) but once their comfortable they'll split apart. Then you'll have the stress from their numbers not being high enough.
If I were in your situation I'd finish of each school and then maybe after they've all settled add your betta.
Feel free to message me any questions, I'll try to help the best I can!
Rummy- Mini Rex Rabbit
Diz jr- Betta
Yoshi- Crested Gecko