You never want to simply toss a pirana in with other piranas regardless of size. If your fish are conditioned to eat live food, this introduction would be considered a meal. Also, there are territory issues these fish will manifest into a problem. While keeping piranas of different sizes is not recommended by me because of the hierarchy, what I would suggest is do a water change of at least 20% before introducing a new pirana in and even go as far as moving objects around to break up the territorial behavior. It might give your fish a chance to survive and be accepted.And wondering if i might be able to toss in a smaller red around 3inches in there with them?