I'm sorry Sabrina has diabetes, but you've found the best source of information on feline diabetes treatment anywhere!
From the ProZinc forum, I see the recommended starting dose is 0.5 units twice daily, every 12 hours. I know your vet said otherwise, but you can certainly work up to higher doses as time goes on.
When to begin is ultimately up to you. It took me a week to start insulin after my cat's diagnosis. I strongly advise starting home blood sugar testing before you begin dosing.
You are wise to be concerned about how her change in diet may effect her need for insulin. You're absolutely right, what they eat does impact how much insulin they need. Changing their food or insulin dose can be very beneficial, but it must also be done carefully. You need to be testing her blood sugar at home.
Testing your cat's blood sugar at home is one of the most important things you can do for your cat. A lot of vets discourage home testing for some reason, and they're absolutely WRONG! You do not need an expensive pet glucose meter. Most of us use a human glucose meter. The one that seems most popular to me is the Relion Confirm, bought from Walmart. I use the Relion Prime (it's a bit cheaper). Testing strips will end up being your largest expense.
(I hate the lancing device that comes with the Relion glucose meters. I think it's really painful. If you want to use a lancing device, I strongly recommend the Accu-Chek Softclix lancing device. While you need lancets, you don't NEED a lancing device; some people like to hold the lancets in their hands and poke their cat that way.)
Please be aware that there are different types of insulin out there and they act differently. I use Lantus, and I'm not very familiar with Prozinc. Have you been to the Prozinc forum yet? Their stickies will guide you. When you start giving insulin, you can make your own daily threads there and people can comment. Be sure to ask any questions that cross your mind. I know this is an overwhelming and confusing time. All of us have been there before, and we're happy to help. You lose nothing by asking. Don't be shy.
You are using ProZinc U-40 insulin. Do your syringes say U-40 on them too?