I don’t know that I’d say it’s the absolute most hazardous thing you could do, but it is a pretty bad idea. In order to do a proper quarantine, you have to have the rat in a completely separate house (or airspace, like if you have a basement or garage that’s on a different air conditioning unit/vent system) for 2-3 weeks. That’s obviously very difficult & not something a lot of people can do, but at a bare minimum, you should at least have them in a different room to prevent any external parasites from spreading & lessen the chance of any airborne illnesses spreading.  Unfortunately, previously getting rats from a particular store doesn’t by any means guarantee your new ones won’t be ill (my first two from a store here were fine, out of my next batch of five, one died within a few days & two more nearly died of pneumonia).

Also, it’s an incredibly bad idea to try intros right away, as it’s very stressful for a new rat to have to immediately get used to a new house, being handled by a new person, AND new companions. I try to wait 2-5 days before handling the rat at all & at least 1-2 weeks before attempting an introduction. The only time I’d ever do an intro right away is if I got new rats & then just a day or so later got more.

—Lisa