Mixing Rustoleum is not a problem.
My local Home Depot offered to mix it (they would shake it rather than stir) but in the end I mixed it myself. I mixed a dark blue and a white to get a light blue, and it came out great.
I mixed it at home in my garage in stages.
Basically, I had five quarts to mix, and only a four quart pot. So, I'd pour some blue and some white into the pot, stir it, and pour it back into the cans. Then I'd pour some more, stir it, return it to the cans etc etc....It took about 25 minutes to have the whole lot mixed together - and of course, with hindsight if I'd had a 2 gallon pot I could have mixed the whole lot in one go.
Anyway, regardless of how you actually mix it - at Lowes, Home Depot, or at home, you can definitely mix the stuff.

Of course, you've got to know what colour you're aiming for before you buy the paint, and you'll have to do some trial and error before you get the exact shade you want, but that's always the case.