First of all, dogs usually get tapeworms from ingesting fleas. When the flea is digested in the dog's stomach and passes through the digestive tract, the tapeworm larvae that were inside the flea migrate into the intestine of the dog. I don't know that it's very likely that Ruby could get a tapeworm from mouthing a rat, though I suppose it is possible she could have ingested fleas from having the rat in her mouth.
Second, though I wouldn't want to replace your vet's advice, I'd personally wait until seeing signs of worms in her stool, or other symptoms that might indicate another parasitic worm infection, but that's just me.
Third, I very much doubt that you're in jeopardy of getting a tapeworm yourself from Ruby licking you, at least not at this stage of the game, especially if all she did was mouth the rats.
As an aside, I encourage all who are curious about parasites to check out a truly excellent podcast I've been listening to for a few days now called TWiP (This Week in Parasitism), which is available from iTunes for free. It's educational, entertaining, and may set your mind at rest, especially about tapeworms (at least in part!).
For the most part, it's actually a myth that tapeworms cause weight loss in people. Most people who have a tapeworm are actually almost completely asymptomatic, and largely unaffected by the parasite. They really don't take enough nutrients from our food to make an impact on our caloric intake, interestingly enough.