Hi Robin,
Hopefully others will weigh in here. For me personally, I don't use commercial hypoallergenic diets. I tell the owner what we're targeting, then if they want to seek out a processed food that avoids that ingredient, they do it, provided the diet they come up with also avoids excessive starch or carbohydrate levels.
Many of my clients simply apply the broad guidelines in Manual of Natural Veterinary Medicine (paleolithic diet; home-cooked diet appendix), and then decide what protein source they're going to use.
I know that wasn't what you wanted to hear, but just in case this is helpful...
Steve