I don't know exactly where you live in your area, but if you can get to Montreal easily, I go to this place:
http://www.birdandexoticvet.com/. They specialize in exotics, and I completely trust the 2 vets will see rats (the third one is very allergic to them). If you would rather not cross through downtown Montreal and would like to go south shore instead, this is another vet I truly trust (Victoria's vet, actually :cheeky: )
https://www.facebook.com/AnimoSante.
I do not believe there are any exotics vets in Lanaudière. I have met a lizard owner at my clinic that came all the way from Joliette for that reason. If there are some, I'd be curious to know, as my parents are in Joliette and I usually take my sick rats with me when I visit and stay overnight.
For my part, I only keep one colony. All my rats, males and females, are introduced and live in my DCN (granted, I never had over 8 rats at a time). I get ALL my males neutered upon adoption, and always have mellow, squishy, relaxed males that are very accepting of one another, and happy to live with my girls as well, spayed or intact. I even saw 2 love stories in my years on living with rats - Picolo and Chatouille would never be apart, and now, Jay and Ãmilie are glued to one another. :heart: I definitely vote for mixed colonies, so if you can, go for it! But I also found that genders like to hang out together as well, so I try to at least keep 2 of the same sex at a time.