First of all, they are just fine outside, as long as you make sure they are vaccinated yearly and dewormed at least every couple of months. If they turn out to be mousers, then they should really be dewormed monthly.
Second, I have been a vet tech for 10 years, and the only "allergy reducing" food I know of doesn't reduce a persons allergy to the cat, it reduces a cat allergic reaction to foods. allergies are to the pet dander, which is created when the cat cleans themself. The saliva dries out the skin, and the skin flakes off, like dandruff.