Mixing piranhas different size

In my experience yes you can mix them (provided the divider time gave a positive result). But before you do some there are several questions: First of all, (1) what's the size of the tank? a good space for all piranhas is vital for them to get along. (2) You mentioned the 3-4inch are more aggressive than the 6-7, does that mean against each other (within their group) or against the other group?

Thanks for replying, My tank size 615 L and what I mean when I said aggressive they eat very fast within their group (live or frozen food), the bigger ones eat slowly

6 in a tank that size will be fine, your problem will be territories like the plants and rocks. I would introduce the smaller ones with a divider or during a large water change / when you take things out or major decor rearrange.

Awesome! Yeah they should be fine. Congrats!