mm not at all.. i heard about peaceful coexistence with 2 and more spilos! there are few serrasalmus that we can keep in group.. gold spilopleura, sanchezi (red spilo), and brandtii!! i am breeding piranahs since years so i know something about it.. i brought also a spilo in the past! pygocentrus nattereri need around 100 liter each.. serrasalmus need around 200-250 liters each but just for those species
300 ltrs each should be fine anyway!