I noticed that you mentioned that your vet gave the cats a 'shot' for the fleas after a bath. The only injection I know of for flea treatment is Program. Program works well with regular Advantage (not the Advantage Multi version) but you don't want to give both Program and Revolution or both Program and Advantage Multi because they have similar ingredients in them and you would be giving them too much of the same thing. But if you wait 3-4 weeks, it would be safe to use either Revolution or Advantage Multi. But you can use Advantage (regular) safely in conjunction with Program.
Program sterilized eggs and keeps them from hatching and works kind of like birth control. Revolution and Advantage Multi do this too but they also kill adult fleas (which program doesn't do) each time a flea bites the cat and then they die off.
I would call your vet and ask him if he gave the cats Program and if so, you could safely give them regular Advantage but not the Revolution or Advantage Multi.
This is how it was a few years ago and I don't think it has changed.
Hope your new kitties will be itch free soon.