Had a friend who wanted a scavenger with his polys and I suggested a lace cat. It wan only in the tank one day before he heard this croaking coming from the tank that night. He turned on the light to see his big ornate gripping the syno by its stomach and swimming around the tank. He could not catch the fish (big, deep tank with lots of hiding places) and the next day found the cat with its guts torn out. It was a decent sized fish, about 1/3 the size of his polys and too big to swallow, but for some reason they took a dislike to it. I think the only scavengers in his tank are some really big clown loaches and some Ancistrus. BTW, his polys also killed several Leporinus that made the mistake of bothering the polys by fin-nipping them.
Also I see you have some congo tetras in your tank. They max out at around 4", so you will have to relocate them at some point. If you want other African fish, try and African Knife, they are nocturnal but quickly aclimate in many cases to a diurnal feeding schedual and they are unlikely to be dined upon by the polys since they have 'sonar' capabilities and would be hard to sneak up on. Plus they're pretty neat!
