[quote=AmyWinter] I would like to start feeding them more real food [/quote]
Hi again Amy-
First of all I'm sure you know that any dry food is a big no-no. The most important dietary aspect of FLUTD (which includes crystalluria) prevention is adequate hydration and water consumption. You can accomplish this with wet food, added chicken broth with additional fresh food.
Meat, in and of itself, is acidifying. Virtually any fresh meat which you add to the diet will help acidify the urine. Stay away from grains (which cats don't need anyway). Dr. Strombeck's diet book (he is one of the pioneers of veterinary gastroenterolgy) has some home-prepared diets specifically for this purpose. You could add that to Lexi's variety of meals as I wouldn't recommend relying on just one food.
Make sure you check a urine pH every few weeks to make sure that your staying in a good range. You can either bring a sample to your vet or check it at home with litmus paper or special test strips. As you mentioned, vitamin C added to the diet can help as can 250-5000mg cranberry extract (which will prevent bacterial adherence to the bladder wall and thereby help prevent further crystals).
This is a great article http://www.littlebigcat.com/index.php?action=library&act=show&item=017
if you want lots more info.
Please keep us informed about Lexi's progress.