Sorry didnt mean to post that right away! I'm Dr. Hartman and I'm here to help answer any questions you may have =)
That link at the beginning is a good article on introducing a new dog to a resident cat at first.
I will say that the fact that you are getting a small breed dog, one that is older, and one that has a laid back personality are all GOOD things for Topsy.
It also sounds like you have someone else at home that will be able to help with controlled interactions between Topsy and the new dog in the house when you get to the point of introducing the two.
Is Topsy an indoor/outdoor kitty - is that your concern that she might run away? If so, does she have a litter box inside that she uses regularly? If so I would consider keeping her indoors more and spending a lot of time with her just BEFORE the dog is to come home, then doing as the article above suggests about separating the two in different rooms so they can smell each other, etc. I might keep Topsy indoors only, at least until you feel that interaction between Topsy and the dog are going ok with them in the house with no separations of doors/baby gates between them, before you start letting Topsy back outside.
I'll be online all weekend, so if you have a follow up question, just type it in and I'll check online periodically throughout the day!