You are getting some good advice, here. Don't make it too elaborate to begin with, and that means keep your medal categories to a minimum. As others have mentioned, you can group the BJCP categories into broader medal categories.
Group the categories however you would like and then see how the entries roll in. You'll never know until you actually see it. Talk to your core guys and get an idea what they might enter. At least, maybe, they can give you an approximate number. That will give you some idea about how to form the medal categories. For example, if it looks like you are going to have a bunch of wood-aged beers, you might want to split that off on it's own or into a smaller grouping with something complementary.
I do not agree with moving the categories around after you get all the entries in. To entrants (and potential entrants), that smacks of manipulation to provide someone some sort of advantage. You may have the best of intentions and practices, but just the appearance of impropriety can torpedo a competition of any size. Set your categories, publish them, and then take what you get. If your categories wind up out of whack, that's just a lesson to apply next year.
The trickiest part might be getting judges. Do put out the word. I bet there are some good judges in your area, and someone might be quite happy to come out and help out with a new, small competition. After serving as the Head Judge for an 800+ entry competition, that sort of thing almost sounds like a vacation!
TL