Taking plague rats or storm fiends as battleline plays different to clan rats. Mobs of giant rats will probably play out similar to clan rats, but with 3" range and more wounds.
I'm under the impression that you only get access to the traits and artifacts specific to your clan (except masterclan generals in clan only lists), though I could be mistaken.
Many WHFB skaven players will already have those sort of numbers of clan rats laying around, so don't be surprised if that's a thing that happens.