I always thought that in terms of skiddishness and agressions the order always went from most skiddish and least aggressive to least skiddish and most aggressive:
Red Belly -> Caribe -> Piraya (I could be wrong) Of course all these factors can be influenced by tank layout, feedings, temperature, etc.
REDs- Skittish and usually very shy.
Caribe-Bold and aggressive, good eaters.
Piraya-Big and can be very aggressive and fight your other fish.
Ternetzi-Related to the Reds but is bigger and at higher tank temps they tend to "flip out" and go on a killing spree.
All are able to shoal but pirayas tend to need space. Reds you can over crowd in big tanks.
they shouldn't .but with p you can never really tell there fussy fish. if they don't like something(even there own kind)they will take them out.but usually if you put them in around the same size move the tank around(so there no territories)they should get along