Many great recipes, both homebrewed and commercial, use multiple varietals for dry-hopping. Without seeing your recipe or knowing your taste, I can't tell you if it's a good idea for your beer. But there's certainly no hard and fast rule against using multiple varietals in your dry-hopping.
I find that Centennial, Cascade and Chinook have similar flavors when used in dry hopping - I mix 'em all the time (depending on which I have the most of on hand).
Mixing varieties gives you a much more complex aroma than using piles of one hop. My favorite combo for IPAs is CPA, Cascade, Perl and Amarillo. A half ounce of each gives an amazing nose.