Well, if you absolutely can't quarantine in another room...at least put the snake on the farthest side of the room from your existing colony as possible. If it MUST live in the same rack...there are still steps you can take to help prevent parasite contamination, but not much you can do about any air-born contagions.

Pre-treat for mites with Provent-a-Mite. If you accidently bring mites in with a new snake, at least you can kill 'em off as soon as they hit the new tub.

NEVER share rodents. If the new snake refuses to eat something, don't give it to another snake. Throw it away or save it for later to give back to that same snake. (Which would mean don't put it back into your existing rodent colony as well, if you have on)

Make sure the new snake has its own sets of water dishes and hides and whatever else you need. Don't share furniture or equipment with the other snakes while in QT.

Always handle the new guy last, when handling multiple snakes in a day.

And of course...wash wash wash!