I have to agree that the number of litter boxes will depend on your cats. I don't go by the rule of thumb because I have 3 extra large litter boxes for nine cats. I just scoop morning, evening, and right before bed. They are ok with it so I'm not adding anymore. I'm like you and don't have space for another one if I can avoid it. You could just try the one and see if it upsets Rizzo or the new kitty. Just be prepared to add one if you have to but I bet one will do fine.
I use freshstep clumping litter found on all Walmart shelves!