Good question. I mix it up all the time. There really isn't any consistency between different hook companies and their model numbers, so if you see a pattern listing a Mustad, and you just have TMC's or another brand it can be hard to know what to use. Here's a conversion chart to see comparable hooks from different companies and what hooks are typically used for what..
Here's a good resource for you:
Fly Hook Comparison Chart
and while I’m there, here’s one for bead sizes and Mustad hooks. Not all hooks accept beads well.
In addition to using comparable hooks for listed patterns, you can also mix it up a little using stuff you have to tie different things. For instance sometimes I'll tie soft hackles or nymphs on a dry fly hook if thats what I have.
What I try to do is to make sure the shank length is a close match to get the size right. As I understand it back in the day they had hooks in different sizes, including odd numbers, which you don't generally see anymore. A size 12 2xl nymph hook would have a shank length of a hook 2 sizes up, counting the old odd size 11, that would make it the same length as a standard 10.
So, some of the "weird" things I do, would be to tie size small nymphs on a size 18 and 20 dry fly hook, which would be the same size as 20 and 22 2xl nymphs, but with a bigger gape for better hooking.
Another example would be our large Eastern Green Drakes. Fly patterns typically call for a size 8 dry fly hook, but i use a size 12 4xl streamer hook. The shank is the right length, but the hook weighs less and is easier to float. (Even though it's a streamer hook, the wire is thinner than a size 8 1x fine dry fly hook). I just use wings the length of the standard size 8 (=12 4xl shank) and size 8 hackle to keep the proportions right.
Another example might be a nymph pattern tied on a 3906 or 3906B. If I want to tie it in a bead head, I use a smaller size 2xl nymph hook like a 9671 because it accepts beads easier.
I just started using a light wire scud hook (TMC 2487) and have been tying a lot of emerger patterns on it instead of a dry fly hook called for in some recipes. It gives the fly a nice curved look, but will still (hopefully) ride in the film.
So feel free to mix it up. As long as you have an idea of what the basic properties of the hook are in the recipe, and/or you know what look you're going for, you can just use whatever you have. If you run out of a size 12 2xl nymph hook, use a size 10 std dry hook and you're in business. You can put a couple wraps of lead, extra wraps of ribbing wire, or a bead on it before building the rest of the fly if you want it to go deep.
If you have a pattern recipe in mind listing a specific hook, and want to know some other choices, based on the hooks you already have, just holler. I'm sure you'll get a lot of options.
Hope this helps.