You've got a few options. You could add something like a pair of rainbows or other small cichlids, and let each pair stake out their own territory.
Personally, I would let the sajica pair be the bosses of the tank and fill up with a few swordtails/platies and/or maybe some bleeding heart tetras. If you want to actually raise the young in there though, it'd be best to have no other fish but the sajicas.
For decor, I'd go for sand or a fine gravel. THen rocks and a couple of plastic plants. But, to be honest, the sajicas won't really care how you do it up. I'd just do it however you fancy it. I think their original biotope would be something like sand/fine gravel with rocks and few, if any, plants.
