as long as you realize the risks and think that you can handle it, i say go for it. there is no such thing as a fool-proof way to add more pygos to the shoal, no matter what you do, there is a big risk involved. there are however things you can do to cut down on those risks, so if you decide to do it, post up and myself and others can walk you through it. i have kept piranhas for years with no serious problems, but i still get pretty nervous when i put together a shoal or add to it. i actually have two new caribes that i'm gonna be adding in with my reds once they are done with the quarantine and as excited as i am to have them in that tank, i am not looking forward to the anxiety-filled first 24 hours or so after i put them in. how big of a tank are you planning to upgrade to?