Not at all. I'd guess the majority of people on BYC have mixed flocks. The only times problems crop up is among individuals--sometimes chickens get pushed around if they're a more passive breed (Faverolles come to mind) and sometimes crested breeds like Polish have their crests plucked by other breeds. Otherwise, just keep an eye on your flock dynamics and it'll be fine!
I have a very mixed flock all sizes and colors. I keep the bantams housed separate from the standards but they range together
