If pirahnas get hungry they will hapily eat each other and SD's are not nerly as equiped for defense as RBP's are the only way you could keep both in the tank would be to divide it some how
This is probably a stupid question, but how do you do water changes on a school of pirahnas? Do you just feed them really well and hope for the best? I just remember my aunt had a red belly and she had to move it from one tank to another and it attacked the net as soon as it was put in the tank and then bit through it and landed on the floor. She ended up grabbing him with two pieces of cardboard and he was fine.
Also, I heard somewhere that the "magic number" for a school of piranhas is seven and up in order for a "peaceful" aquarium, has anyone else heard this? I think maybe I read it somewhere...