There shouldn't be much more in a 55G than a big oscar anyways...depending on filtration of course. It is true, as the RD gets bigger, nothing else will be able to live with it in a 55g...that's my past experience anyway. Even just leaving out the RD for now could work temporarily. Salvin's cichlids are nasty and can hold their own...though if a pair forms, the odd man out may take a beating. The salvini would outclass the blood parrott too...nasty weapons on the salvini. If you really want a mix of these fish, I'd have the oscar, and perhaps try the salvini, but leave out the BP and the devil. Not the ideal setup, but may work and be an interesting tank to watch.
