I'm gonna skip the size requirements on this question, becuase their are more than enough overstocking police on this site to cover it, and just answer the question before the thread gets sidetracked into a thread about why pacu shouldn't be sold...
So..
That really depends on the situation,
Yes, if given an incredibly large tank, and no spawning "aggresive cichlids"
No if your talking really nasty fish, Buttikoferi, Dovii.. these types, fish with a nasty disposition, who will mark out a territory larger than most public aquaria can provide..
Most large cichlids.. including the "evil and aggresive" jag, can be quite tame if given enough room, some fish, really are just plain evil though.
No, if the tank is smaller than a few thousand gallons, and you have a pair of spawning cichlids on your hands.. then your gonna be in trouble, if they don't kill it outright, they will certainly stress it to the point of infection, disease and eventual death.
Make sure all the cichlids are the same sex, which is pretty tough, if you wanna add a monster like a pacu, because any pacu large enough to not be lunch, is not gonna have anywhere to hide from a large angry bruiser.