Can I use advantage one week after frontline plus?

We accidentally must have brought fleas home when we played with those shelter cats at PetsMart... now my 12 year old cat that hasn't had fleas since we brought him home when he was a kitten has fleas all over him. I found 20 fleas on him one week ago, about 3 weeks after we played with the shelter kittens, so I treated him with Frontline plus, sprayed the house with Adams Flea and Tick spray, and I've been vacuuming twice a day underneath the furniture, in the crevices of the couch, etc, and I've been brushing him with a flea comb trying to get any adults. I've only seen 2 dead fleas this whole time. And I found 6 very alive ones yesterday. Why isn't any of this working? I have heard that Advantage works better for fleas, so I was wondering if it would be safe to apply Advantage to him now... one week after the Frontline Plus which does not seem to be working.