Introducing new foods to a cat with allergies

He's been eating EVO 95% venison successfully for 9 weeks now. No more diarrhea, vomiting, and he goes in the litter box 100% of the time now. I want to try some other "novel" protein sources, like duck, and also try to see if he has a grain allergy, or if it is just the chicken. I'm not sure how to go about trying these things. Would I have to spend another full 8 weeks feeding the duck, or adding in rice? Can the duck be added in addition to the venison or should he be switched over completely for the time being? Should I even bother since he's doing so well on the venison? Also, when switching between brands or flavors of wet foods with a cat with allergies, should you gradually change foods, or can you just switch wet food flavors with them like you would with any other cat?

Lots of questions, but any help is appreciated. Thanks in advance!

IMHO and this is based on oriental medicine and the heat of foods so take it as you want ..

I would introduce lamb for about a week ( you should see a flare up in that time ) and then each week add a new meat .. I would do lamb , beef , then a poultry like duck or turkey ... after you have found the best liked and tolerated meats then if you want add grains... IMHO with allergies I would likely add "exotic" grains like quinoa or orzo( this is a wheat but an ancient form) , kamut , millet and stay away from the common ones for a few more months if you can ... With my allergy kids only one gets any kind of grain and that is cause she has some protein ( meat issue)...

Before the z/d and before the venison we did try Nature's Variety Prairie Lamb and Rice, and it was just as bad with that as he was on a grain free chicken food (which was before we suspected food allergies). So basically, we don't know if it was the lamb or the grains that were the source of the issue.

Also, when you suggest adding a different meats for a week each did you mean fresh meats or grain free canned foods with those as the sole protein?

I would do fresh meats with organs... but canned is likely to " be 100% complete and balenced "

I have to say there are a few grains in the Prairie.. I will highlight the "probable " allergens I have a probale cat and a non probalble dog
http://www.naturesvariety.com/prairie_cat_kibble_lamb
NGREDIENTS: Lamb Meal, Canola Oil (preserved with Mixed Tocopherols and Citric Acid), Oatmeal, Brown Rice, Ground Flaxseeds, Montmorillonite Clay, Natural Lamb Flavor, Alfalfa Meal, Menhaden Fish Meal, Lamb Liver, Potassium Chloride, Sea Salt, DL-Methionine, Vitamins (Choline Chloride, Vitamin E Supplement, Ascorbic Acid, Biotin, Niacin Supplement, Vitamin A Acetate, D-Calcium Pantothenate, Riboflavin Supplement, Pyridoxine Hydrochloride, Ethylenediamine Dihydriodide, Thiamine Mononitrate, Vitamin B12 Supplement, Beta Carotene, Vitamin D3 Supplement, Folic Acid), Minerals (Zinc Proteinate, Iron Proteinate, Manganese Proteinate, Copper Proteinate, Sodium Selenite), Taurine, Dried Enterococcus Faecium Fermentation Product, Dried Lactobacillus Acidophilus Fermentation Product, Inulin, Flaxseed Oil, Apples, Cranberries, Freeze Dried Lamb, Freeze Dried Lamb Hearts, Freeze Dried Lamb Liver, Pumpkinseeds, Carrots, Ground Lamb Bone, Butternut Squash, Chicken Eggs, Broccoli, Lettuce, Spinach, Dried Kelp, Salmon Oil, Apple Cider Vinegar, Parsley, Honey, Blueberries, Alfalfa Sprouts, Persimmons, Olive Oil, Duck Eggs, Pheasant Eggs, Quail Eggs, Sage, Clove, Rosemary Extract

there are two grains and two grain likes that I would suspect before lamb ...

all the instict cans have flax
http://www.naturesvariety.com/instinct_cat_can
but are complete..

the 95% evo s are "complete" thus no worry just feeding one and they are much simplier than instinct ...