I think getting a companion for Loki is a great idea, and I'm glad it was so easy to find him one. As for the vaccines, you will have to ask the shop what their policy is, but the current recommendations for shelters is to give a vaccine at 6, 9, and 12 weeks (there are different recommendations for pet owners). I would definitely try to keep her in isolation until at least a week after the first vaccine, but preferably until after the second one.
Also, remember that Mary Ann will most likely be viewed as an intruder by Loki, especially if he is so attached to you and he thinks she is taking your time away from him, so you may need to do a bit of work on the introductions once she is out of quarantine.
Also, remember that Mary Ann will most likely be viewed as an intruder by Loki, especially if he is so attached to you and he thinks she is taking your time away from him, so you may need to do a bit of work on the introductions once she is out of quarantine.