I have heard--and I have used--mixed time signatures before. Here you're in 4/4 time and then you have one measure of 2/4 time and then back to 4/4 time.
I've even seen songs vary from 4/4 to 3/4 or vice versa. And I've seen songs switch between 6/8 and 3/4. You can do pretty much whatever you want, really.
Vicki