as long as the cloudy water is only aestetic.. Uaru... otherwise address the water first... adding new fiht to a potentially compromised system to begin with could lead to bad things happening. Uaru are pretty interesting and remind me alot of discus ( no where near the colors but have pleasent personalities), but are generally docile enough to be kept with other delicate species. one of thes few fish I've seen kept with rays and not attack them, so most likely just fine with your stock.
I agree with looking into the water quality/clarity issues before adding anything to the tank.
What size is the tank?
I kept Congos with Angelfish, Clown Loaches, Roselines, Pink Tail Chalceus and some Madagascar Rainbows and never had any problems with the Congo's nipping fins. They would spend all day chasing each other around the tank, but never to the point of shredding each other's fins.
Water quality is fine. I'm pretty sure I need to add some extra filtration with 25 micron filter pads. The only filter pad I have is in the wet dry and that catches only large particles at a flow rate of 4K gallon/hr. Tank size is 500.
i would go with a rotkiel severum, I have one right now. It has great colors and it never bothers any of my other fish(gar, knifefish, clown loaches, and a stingray)