We recommend only three insulins for use in cats: Lantus (glargine), Levemir (determir), or PZI/ProZinc. Other insulins
will not work effectively with a cat's faster metabolism. Please let us know what insulin they prescribed and what starting dose. We've seen quite a few newly diagnosed cats started at way too high of doses. 1u given twice a day is the usual starting dose, so anything higher than that or anything not dosed consistently (i.e. once daily dosing) might be problematic.
We also recommend a
low carb diet of less than 10% carbs (the lower the better). Friskies Pates, Fancy Feast Classics, and Wellness Grain Free are some commonly used commercial foods around here for diabetic cats. They don't need a "special" diet; just low carbs. These foods are also perfectly safe to feed to your other cats (if you have any).
Finally, we are huge advocates of
testing your cat at home. This will give you a better idea of how your cat is doing on the insulin dose and it keeps your kitty safe from dropping too low or remaining consistently too high. No fancy and expensive pet meter is required and most of us here use human meters so we're well-versed in accounting for the "discrepancies" between a human meter and a pet-specific meter.
All of this information is contained in the
AAHA Guidelines for Diabetes from 2010, so your vet should hopefully be up-to-date. If not, I would bring a copy of this with you to the vet visit in case they want to prescribe you an outdated insulin or try to discourage home-testing or are pushing (always) overpriced and (usually) high carb "prescription" food on you.