Hello Jojo321, thats not a problem I've ever had. I feed a dried complete kibble as the basis of their food, but also feed a little of what we have to vary it. No matter what food manufactures tell you about feeding different foods leads to gippy tunnies, no dog in the wild eats the same food every day! I find that by varying the food my dogs have cast iron tummies, can eat just about anything, including Chinese sweet and sour chicken and Spag bol!
It depends on what their allergies are related to, mine don't (thankfully) have any, but they can be allergic to various ingredients in food, dust mites, storage mites, grass etc. It would seem a lot of dogs are allergic to chicken in processed dry dog foods.