It sounds like you need to talk this through with your vet. I have been doing research on the internet to better understand this area, and I found this pdf (attached) that explains it pretty well. You may already know all this but it is new to me so I am sharing in case it is useful to you as well. :-)
It appears that the z/d and other diets given to pets with allergies may contain the food that our pets are allergic to, but the protein in the food (chicken for example), which is what our pets might normally react to, is broken down into smaller proteins (i.e. hydrolyzed) to make it somehow less allergic. Also z/d etc. add ingredients that are anti-inflammatory.
The article attached suggests that the best course of action for pets with allergies is to find a food that they have never before eaten, because it has the least chance of provoking an allergic reaction. This probably explains why my Curly does not seem allergic to turkey, because he probably never had it before.
I did see that Royal Canin also has veterinary prescription diets- maybe one of these would be preferable to the z/d(?) and you can talk it over with your vet? I don't know if they have a hydrolyzed wet food but they seem to have a long list of veterinary-prescribed diets. One of the hypoallergenic foods is rabbit and green pea which looked interesting to me. Here is the link in case you end up needing to try this area http://www.royalcanin.us/cats/Veterinary_Diets.aspx
H2-4_Food_Allergies.pdf 591k .pdf file
You probably also have seen my links for the allergy pills I am using for Curly. They have helped him tremendously but it took a while for his fur to grow back and inflammation to subside. When I took Curly to the vet, he had gouged his neck pretty badly (he is a 4-paw polydactyl!) and had scabs all over his neck and behind his ears, so the vet gave him a steroid shot. This may not be the best thing to do on an ongoing basis for a cat with FIV, but Curly's symptoms were pretty intense and the vet said it is better than the stress of his itching and his immune system going haywire with the allergy. My feline FIV support group also confirmed this, and the head of that group is a resource around the world for his collection of research in, and knowledge of, treating cats with FIV.
I started giving Curly his allergy pills at the same time he had the steroid shot because I didn't want to have to give him more shots. After the first shot, Curly did not need any further shots and he responded to the pills, probably together with his food and the shot. It did take at least about six weeks for Curly to really show improvement, during which time I was fiddling around with foods. At a certain point I realized that it was most important to give him food he would not react to, and get him stable and fortify his system with the pills which also contain vitamin C which he needs being FIV+. Skin conditions are common in cats like him with FIV.
I don't know what advice to give you for Lucas other than sharing your concern that the z/d hydrolyzed chicken might not be the best hydrolyzed formula since Lucas seems to be allergic to chicken, and asking him about trying a different hydrolyzed formula. Let me know if you need the link for the allergy pills. If I were you, I would maybe try the rabbit and pea food, or maybe a lamb and rice food. BG brand food is really good, you can buy it at PetCo, and they offer a lot of very unique wet foods. The BG beef does not show any other ingredients besides beef- unfortunately Curly reacted to it.
Last but not least, if you have a Petsmart, you should get those Whisker City hypoallergenic cat wipes. They are all natural ingredients and you can wipe Lucas' tummy with them, it should give him temporary relief.
). If you don't see any improvement after a few days, you can bump it up to half pill in the morning and half at night (Assuming Lucas weighs about 10 pounds or more).
