I'm not a big fan of larger cichlids but you can't keep the firemouth eith the smaller rummy's and cories. Not sure about the severum so I strongly suggest either posting this in the cichlid forum where all the cichlid people are or doing a lot of reseach before buying anything.
As for ph, what your read on species information is what the preferred or wild ph is, depending on which site you look at. BUT if your getting them from a LFS most likely their water is close in readings to yours and the fish are already acclimated to a higher ph. I've been in this hobby for well over 30 years and the important thing is that ph is constant no matter what the level is. If you try messing with it you being to make it unstable, jumping it up and down, and that is very hard on fish. Unless you commit to using RO with tap water every single WC so the ph stays constant then just leave the ph alone or risk losing the fish you do buy.