@Sglass,
Seaglass
You mean the flea stuff (in little tubes) that you apply to the back of the neck/head? That is supposed to do the trick, if used regularly.
Feeding Claws inside would be a good idea. At least you'll know the food you're supplying is actually going to the right place. When you say "feral" do you mean that Fluff has no one else looking out for him/feeding him, etc ..? <sigh>
I suspect (sigh) that Claw's fleas are not going to vanish until Fluff's do, if they continue hanging out together. <sighing again> ... & until the house is de-flead, too.
Sadly, if Fluff learns to depend on you for food, you don't know if the house owners will continue to oblige him. (It'd be wonderful if they were interested!)
um .. you wouldn't be interested in adopting Fluff yourself, would you? He sounds a pretty good cat to me.