IMO and IME, a pair with just 1 other large fish has very low odds of succeeding over the long term.
Some sort of stalemate for a while, might be confused for "success" by some aquarists, but is generally unlikely to last for very long. When a tank gets divided in 1/2 between apposing sides it almost always eventually escalates. Gets very violent.....and one side prevails!
If you want to do a pair with other fish, you need a lot more fish then just one. You need lot's going on in the tank. An actual working community. You may not want to stock that heavy and a 75 gal. may not have the room, especially if it involves larger fish. Then stock just a pair and use a divider. And yes, a CA pair kept with out tank mates, a divider should be employed most of the time, particularly between spawns. Hiding spots that only the female can fit or a hole through the divider that only allows the female to escape can be utilized as well, but generally shouldn't be relied upon solely as they may need to be completely divided at times for both to flourish.
Rainbows can definitely be kept in groups. Start with 6-10 and maybe eventually extra males might have to be removed (either an aggressor or one getting picked on). Though it is not unusual for groups to work very well over the long term with out ever having to remove from the group. Firemouths as well, may people have had success keeping them in larger groups.
Larger groups of cons, IME, usually works for some time. A couple years. Pairing up with females allows subordinate males to be some what more even with the more dominant males....though likely for one male to eventually dominate fully and the subordinate males having to be removed.
IME, a lot of big cichlids will come to ignore being bitten on the flank by a female convict. I'm sure it doesn't tickle but even a large female con doesn't really do any damage to a big CA cichlid. They learn that it is futile to try and dodge a bite on the flank as usually by attempting to do so, they get nailed by the male con a split second after. Essentially getting bitten twice and not even answering back if they miss their target. By focusing on only one at a time, big CA cichlids can defeat cons if they are really intent on doing so. But I've never had a fish that could ignore a 7" female salvini swinging around and nailing in the flank. I don't think there's a cichlid that could
That does damage like big CA. Can take scales and even a chunk of flesh!
I wouldn't put any faith in the CA/SA cookie cutters. A few of the SA might be OK, but CA cookie cutter, IMO, is absolute garbage. I believe they were concocted simply to have something listed under a category and were never tried and tested in the real world. Made up in somebodies head.....not actual aquariums that were ever set up this way for many years. A pair of salvini and a pair of cons in a 55 gal.
IME that doesn't work long term in a 180 gal., let alone in a 55 gal. IME, sals will fight and fight cons until they lose. I've had salvinis going on 18 years and have always kept them with convicts at various stages, sometimes in many tanks. Young small fish or females of one kind with male or pairs of the other, there is always some conflict and works so-so but can co-exist that way. Full grown male sal with full grown male con, unless there is something bigger and badder in the tank to keep the male sal in check, IME, that don't work. Full grown pairs of both in the same tank, IME, that doesn't work at all. IME, too violent and the cons will lose.