i try and avoid introducing lone rats waiting until i can add a pair to my group. much kinder on them long term. Plus i do the carrier method so rats are introduced then stay together from that day on. I'm not a fan of long drawn out intros.
I'm worried about the same thing. My rat is too young to be introduced safely, and I have her in a temporary cage, but I've been feeling so guilty. I put my youngest rat in with her to keep her company because she was so easy to introduce, but then I felt bad for my other rat for having to go into a smaller cage, so I took her out.