You're going to have to match temperaments. Medium sized pikes are voracious predators, hence the stream lined body and large mouth. They will eat anything large enough to fit in their mouth. They typically will be quite territorial as well. In a 50 gallon, you're not going to have much wiggle room for larger semi-aggressive cichlids that would be able to live with a pike.
Pacu (especially black pacu) get huge. They honestly have no place in the pet trade.
Red Devils are highly territorial and one of the more aggressive cichlid species. I honestly wouldn't put a single devil (without any tank mates) in anything less than a 75 gallon. Again, that's by itself.
As for the pike, you could try adding some bottom dwellers such as synodontis catfish (eupterus is particular, although they do get quite large) or hoplo cats (South American, very cool catfish genus, pretty well armored). Avoid any small fish, such as tetras. You could try tinfoil barbs or silver dollars (not in addition to the catfish, one or the other). You would need to get them close to or larger than the pike, and could do a group of three of either fish, as long as you kept up on water changes and were prepared to remove, should things head south
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