A single adult Red devil is perfectly suited to a 75g. Whilst young you could keep some targets with him, i'd go with giant danios (quicker than most), yet even they will disappear sooner rather than later. A larger Pleco could be an option providing there's plenty of cover, again whilst the Rd is young. (Recently i attempted to keep a 4" common Pleco, then a 7" Gibbiceps with my Rd and neither managed through the first night.) Maybe even a small shoal of clown Loaches would work well for you.
Although they are primarily carnivorous, it's a good idea to offer some vegetable matter (shelled peas are good). I always steer clear of both feeders, for obvious reasons, aswell as beefheart which is just to messy. I see no need for either, there are plenty of other foods which are better for them. I just go with a quality pellet, various frozen purchased from a lfs, fresh prawns and fresh fish fillet purchased from the seafood co-op.