The tetras will end up being a meal for sure, the only fish I took out of the tank before putting the P in was an electric blue dempsey. The interesting part was yesterday they were all swimming together in a group, like he was just one of the guys. I added a handful of guppies when I put him in, all the tetras went after them.
The other piranhas that came in as elongatus looked more like rhoms to me, but they were VERY mean for 1.5-2" fish. Then again the LFS rarely has baby rhoms so it's hard for me to gauge their aggression at that size.