3 pirayas x 4 Terns x 3 Cariba's - 240G 2 years (6" - 12")
9 Piraya's x 7 Terns x 6 Caribas - 720G 3 months (5" - 13"/14")
There is no problem in keeping a shoal in my opinion.. The only issue I ever had of cannabalism was when a Cariba seemed to be getting sick/old (??) and all that was left was his head.
The risk comes with the territory - I don't see there being much risk though as long as you keep an eye on the shoal and remove sick/problem fish when there is an issue.