We have a senior Pug dog with health problems (spondalosis). Her condition is causing her to lose her toilet training abilities as well. I was kind of unsure about getting a puppy, but bringing Missy home has done wonders for Tang. It's brought some spunk back into her, and she even plays with Missy on occasion. We had no problems at all introducing the two. Tang is pretty laid back though. The ONLY issue she has is that she will not let Missy eat her food until she's done with it, which is understandable
I'm not sure if there would be a difference with an abused rescue, I guess it all depends.