One of my rats, Brooklyn, passed away in an accident last weekend. This leaves me with Poppy, an older rat with a tumor. She's not what I'd call healthy, but I don't panic about coming home and finding her dead (a lot). I also have Roo, a younger, somewhat skittish rat who was adopted at the same time as Brookie and may or may not have been related. I'd like to get two more rats so that Roo has friends that are willing to play, but I'm worried about them bullying, injuring, stressing out, or maybe even killing Poppy. Should I get more, and if so, how should I go about introductions to minimize stress?