Can you mix all chicken breeds?

Yes you can easily mix them. I have 2 cream legbars 2 blue bells 2 white sussex 1 cuckoo maran 1 black rock 4 ex bats all living together happily.

It's even easier Looe Looe if you can buy them all from the same place and they are already used to each other.

If they come from different places, it sometimes takes them a while to get used to 'strangers'!  But My layers have been added to over the past 3 years, and Joyfull just lets any new chickens straight into her garden and at the end of the day they usually just follow the others into one of the coops!

Hi, just a word of caution!  Though we have a lovely & harmonious mix in our back garden we did have a few glitches when introducing some new ones once.  The established (quite feisty) 5 girls were not welcoming of the new 3 at all and although we had kept the two groups within sight of each other for two weeks and had gradually moved them closer and closer the initial mix was a mess with the 3 new girls beaten up good and proper.  We supervised the mix and intervened to break them up for a few days but on the 1st unsupervised day one had to be put down she got so badly pecked.  We obviously separated them again and did another long intro which worked and they live together v v happily now.  One theory is that the most bullied bird had a weakness we were unaware of but the chickens could detect it and refused to accept her.  Even our vet said it was unfortunate but chickens will be chickens and sometimes brutal.
hope your mix is successful!