I realize on the surface this may seem like semantics, but the term "predator" is often misunderstood.
A 2" cardinal tetra, is no less a predator than a piranha, they just predate on smaller items.
So for your size tank, only one or a group of very small predators would be proper.
Guppies are as much predators as most cichlids.
Most cichlids, are not really predators, they are omnivores, meaning they are opportunistic feeders that get often more than 50% of their diet by eating plants (including algae). And there are a number of cichlids that are predominantly vegetarian.
Or ...... by predator do you mean piscivore, (fish eater). Piscavores are even fewer in number when it comes to cichlids, many omnivores will eat a fish if they get a chance, but many cichlids in nature are inept at chasing down fish, the exceptions are Cichla, Parachromis, and Petenia and a few others from the new world, but all these get too big for your tank, so until you get something over 100 gallons stick to fish that don't get any bigger than 2" or 3".