^^^ you just never know until you try...
here's my take on it.....it's going to be trial and error and just because it works this week, doesnt mean it'll work next week. With cichlids, you never know. i have a 90 gallon that i'm having a hard time keeping a 3"convict, 4"firemouth and a 4" white saum. Unfortunately, I'm probably going to have to rehome somebody, because the firemouth is just an ass. Every fish is different and every combo is different as well.
Ironically enough, this same tank at one point housed those three fish plus a 5"jd and a 5" Midas and even though it was less problematic, it was far from a relaxed and easy going tank. Cichlids require space and some fish of the same species will require more than others. Personally, I think if you try to house any other fish with a salvini pair in a 55, it's eventually going to be a war zone. Pound for pound, salvinis in breeding mode are some of the most aggressive cichlids going. just my 2c of course.....
Id pick a pair of something you like and add some other non cichlid fish to the mix....