A 75 gallon is the bare minimum size tank for a pair IMO. Some of the species of Heros are large-growing and would not be suited to a tank that size. The problem that you run into is that breeding pairs of severums can and do often have disagreements when spawning and raising fry, especially if there aren't dither/target fish in the tank for them to take out their aggression on. What can happen is that the pair will turn on each other, usually the male on the female, and you end up with a badly battered or dead fish. I have seen this happen more times than I can count, and that is the reason I no longer isolate cichlid pairs into "breeding tanks." Instead I let them pair and spawn in the community tank, then siphon out the fry when they hatch and raise them separately.
For now you'll have to focus on growing out your fish. Try to get an even number; four is the minimum I'd try. This will prevent aggression and bullying from a dominant fish onto a submissive fish, which happens sometimes when you only have two or three. It can work with two or three, but it can also turn into a problem as they settle in and one fish becomes the boss. Having four will also increase your chances of a mix of males and females. You can always rehome some later.
Heros do eat plants, not just when spawning. They are an omnivorous fish and as they mature will likely eat most types of vegetation. Those that they do not eat they may decide to shred and rearrange for fun. Some people have had success with anubias, java fern, or thick, large Amazon swords. I personally have never had it work. My large gold pair destroyed Amazon swords. All of my Heros routinely eat the roots off my pothos vines and then drag the vines into the water and shred them.
All Heros species, with maybe the exception of the mouthbrooding H. liberifer, can inter-breed. Golds and greens can be bred. Personally I wouldn't do this. If you plan to raise fry to eventually sell or give away, you don't want to give people mutt fish if you can help it. Identifying pet store bought severums is already difficult because of the interbreeding of all the various types, so allowing them to do this only creates more confusion.
IMO you'll need more than a single 75 gallon to raise fry. At a certain size they usually start to annoy the pair, so the pair sometimes eats them, especially if the pair is ready to spawn again. To get them up to sellable size (2" or so) you'll need lots of space and clean water for at least two or three months. If you only raise a handful at a time (50 or so) it might be doable in a 4' tank but if you are raising large spawns (my last H. severus spawn was 215 and that was only the half I saved) you'll need more. Those 215 fish were divided among two 75 and one 150 gallon tanks by the time they hit 2".
Hope this helps.