Rule of thumb for cats - one litter box per cat plus one extra.
Welcome and we need to see pics please.
FYI - Introducing cats properly in the furture will probably also cut down on the fighting issues. You could try some tricks now to see if they will work.
Baby powder rubbed on your hands and then on the cats so they all smell the same.
Separate rooms so they can smell but not see for a while.
Trading sleeping blankets, clothes, whatever.
This is a terrific site for learning more about managing a multi cat household. Never too late to learn.