I'm not sure I get the question. Rats and cats, if you go back far enough, have a common ancestor. That ancestor, however, was neither a rat nor a cat. AFAIK, cats did not diverge from the rodent group (or vice versa).
So, how would you say that two carnivor mammals do not share the same lineage? And that mammals did not start out small, and "rat-sized"?
All those things are true- all mammals share a common lineage and they all come from small mammals. However, those original mammals weren't really cats, dogs or rodents.
OK, I'll allow that explanation. (again, Laymen's)
Lets go with... cat (earliest carnivores) to bear.