I've kept severums for years in pH 7.6 or so and moderate hardness. In fact that includes a wild rotkeil I have that is now 12 years old, maybe a year or two older (got him as an adult 10 years ago). So you should be ok there, at least from my experience.
Probably not a standard, 100% reliable answer to your question and I'm not going to contradict what someone else says is their experience. For one thing, severums can vary quite a bit in temperament, so this can depend on your particular fish. But my experience is it's very possible to keep a group of them together. I've done this with anywhere from two to six adults and a bunch of juvies, in 4 and 6 ft. tanks. IMO it's possible you could have two pairing up, but not necessarily a breeding pair at this point. You could also just have two that feel compatible at this point, but which may just as well not like each other by the time they're actually ready to breed, which is probably yet in the future at their present size (assuming they've grown normally).
Possible adding more tank structure in the form of plants, wood, etc. will help, but also possible it won't make much difference and what they really need is a larger tank. Possible other fish will help, but not necessarily. Possible they'd actually better do in a larger group, at that size I've kept them in groups of 10 or more in a 75 gal tank, which worked pretty well until two pairs started to form at about 6 inches, which suddenly turned the 75 small. After that I traded some and put six of them in a 135 gal tank and problem solved.
These are just suggestions. Again, my point is since they can vary a lot in temperament and there are a lot of other variables, what works for you may be different from someone else's experience.
