I keep my tiger barbs in a group of six and they don't really annoy or bother any of the other fish in the tank. I'm pretty sure you rummy-nose tetra will be ok. Just make sure that you have enough hiding spots...so they can hide just incase the tiger barbs get a little mean.
Your 3ft tank won't be over stocked, well that's what I think. Frequent water changes will help though.