Moosmom covered it! Can't emphasize the litterbox thing enough. Everyone with their own box avoids ambushing and territorial spats around the kitty bathrooms.
The rest of her advice should set you up for a successful merge of the new guys into the household. And yes, your cats will let you know how the pecking order is going to go.
We have five boys and still have to revisit who's top cat every once in a while, but nothing serious. They just work it out and get over it.
Good luck and thanks for taking in adults who otherwise would be in a shelter somwhere.

PS Try Feliway spray if things get too tense. Has a calming effect so they say. We used it when we rescued Norbert (the blk/white) and he and William were not hitting it off. It helped a lot.