[2nd2none]

it *might be the food, but since you feed the 2 dogs different food, it’s unlikely they’d have the same rash.
have you had either checked for mites? a skin scraping will tell, especially since, as you termed it, the older ones’ is “more severe”. might warrant a visit to your vet before it gets too serious, hence, more costly.

see if this vid will help you decipher the type of rash:
http://pets.webmd.com/dogs/ss/slideshow-skin-problems-in-dogs

been into the ‘breed’ (APBT) for over a decade and still own a few