Can I use half a dose of Frontline Plus on a kitten under one pound?

I wouldn't. If you are taking this kitten in and accepting responsibility for it then you need to take it to a vet. It needs a wellness exam, the vet can tell you about how old it is, it will eventually need shots and worming, and spayed/neutered. If you don't plan to do any of that or you don't plan to keep it, take it to a rescue or no kill shelter. They will vet it and place it with a foster until it's old enough to be adopted out.

No the real issue here is age. It is not appropriate to put any over the counter flea medication on a kitten that size. Either take her in to the vet who can flea treat her, or get a flea comb. You don't need to bathe her - put a couple drops of dishwashing liquid in a cup of warm water. Comb the kitten both with and against the lie of the hair, dipping the comb in the water every few strokes. Get the comb down to the skin. Repeat daily. No over the counter flea medication - not powder, nor bath, or drops - can be used on a kitten that size and age.

Call your vet and ask him.

Never use a flea collar on any animal.

Get a flea comb and bathe her. Use the flea comb to remove fleas. She sounds like she is too little to use that kind of flea repellent on.