One by one is probably a lot less stressful/overwhelming for them.

I think its smart to be cautious with introducing your doxie to the bigger breeds. I also agree that BSL is wrong, but Pit bulls have been bred to fight over dogs. I have heard many instances of spontaneous dog aggression, but then again I've heard many more stories of successful "co-habbing". Just to be on the safe side, I would supervise all interactions in the beginning and separate your dog from the others when you are not home. Sometimes groups of dogs form a strong "pack mentality" and you wouldn't want them mistaking yours for prey (though I doubt that would happen).

Good luck with the introductions....and the possible future addition?