Holy WOW i was thinking the same thing thats a lot of neon's !!! They wouldn't last a minute in my tank. . . My p's eat thouse large pets mart feeders. . . the 2inch ones by the dozens..... i put 24 in last night and there are only two left this morning. thoes tetras are like a dollar a piece. . . anyway i have kept dozens of piranha over the days and the most active ones that i have kept were by far the redbellied pygo. species. I know if importracer were to respond to this he would tell you the same thing. In my experience the serr. species tend to hide out a bit and chill until feeding time. but the reds seem to cruse the tank a bit more in the off hours. especially in a shoal . . . i break the rules though and have great luck keeping both the pygos in with the serr's. 
i know but its worked for me for some years so i'm sticking with all the same fish. hope it helps.
i know but its worked for me for some years so i'm sticking with all the same fish. hope it helps.