If youre still looking for nice fish to go with pseudotropheus demasoni i would suggest other pseudos like p. flavus.
Just about everyone will tell you labidochromis caeruleus will be fine too. generally with demasoni in mind you want to be looking for smaller mbuna. if you can get your hands on any wild caught or f1 afras i cant reccommend them enough, there plenty of different colour forms from different areas, jalos in particular are stunning.
fish i'd avoid are big things like some zebras, agressive things like melanochromis chipokae, metriaclima lombardoi and pseuodotropheus socolofi.
it may seem absolutely crazy to you but species only tanks make excelent show tanks too. you could easily get 16 demasoni in a 75G tank.
as for catfish, syno petricola, polli, multipunctatus are all brilliant social catfish that wont hide in the dark all day. not bad at keeping snails down and general un eaten food eating.
if you want a pleco go for bog standard tank bred ancistrus, they are hard as nails and efficient algae eaters, you'd need four or five to make much of a different in you tank, the cichlids generally leave them alone.