I've had two mated pairs of sajicas, and both pairs had that huge difference in size.
Because of that difference, I always had a few pieces of PVC in the tank, the female could escape into if needed.
Discus are nice but you'll start getting bored by them. Cardinals and shrimp are too easy a snack for most fish. Dwarf cichlids like Crenicara, dicrossus, biotecus, apistogramma, microgeophagus, taeinacara, latecara, cleithicara, and nanacara all can work. The sajica, on the other hand as many have said, will not unfortunately. Whatever you decide, good luck.