What are the dimensions, just with the Angels you want to have a tank at least 18 inches tall as they get tall and can graze their long ventral fins on the substrate. Its also just fairer on the fish to let them move up and down freely which if the tank is 12 or 15 inches tall not perfect, the
I think the 50 is about the minimum size I'd be ok with for a Liberifer I had mine in a 65 gallon for a few years and it was ok, but she was much better in my 135 when I upgraded so might be something to think about for the future?
In terms of tank mates they are not overly predatory, but no small tetras etc. Decent sized disk tetras are a good option, I had Lemons with mine for quite a few years. Other characins like Headstanders, Hemiodus, Hatchet Fish would be a good call. Any sort of South American catfish but not the smaller Cory species, Flagtails, Wood Cats, Hoplos, Pictus would all be good. Plecos would be ok but I prefer Whiptails as they are smaller and produce less grace and things like Spoonfaces, Sturisoma and Lamonicthys are stunning species.
I wouldn't add any other cichlids here, not sure on the number of Angels but I'd give the Severum as much space as possible. They are a stunning personality and will become a real pet, I had my Lilly for 8 years and I her previous owner had her for 4 and he bought her as a wild caught adult so she was over 12 years old when she passed in 2019. Absolutely gutting but I knew I'd done my best for her.