With corys it's not just size, there are smallish fish you can often keep with fronts, but cory behavior itself is likely to get them picked off as snacks, besides the water differences. Don't know about petricola if you're wanting your fronts to breed... most people find out they're good at stealing/eating eggs from fronts. Have you considered Gnathochromis? Pretty much a Tanganyikan version of a geophagus.
But, what a lot of people don't realize is fronts will spend some time sifting sand, especially when they're hungry. In the wild, they're not the exclusive piscavores some people think, but will also eat shellfish, crustaceans, etc. and you can see this in their behavior when they scoop mouthfuls of sand looking for food. In any case, if you feed them properly, don't overfeed, there's not much need for a clean-up crew, other than if you want to add the variety of fish to the tank.
