Lots of people do keep mixed breed flocks. I prefer single breed flocks.
What I would suggest is if you plan on having six hens then two of each breed, or all different breeds may save you some social problems.
Try and look for breeds that are known to be breed tolerant.
Try and avoid the situation where you have more than one of a particular breed in an otherwise mixed flock.
Finally beat in mind that having hens of similar body weight can help to avoid bullying.
What I would suggest is if you plan on having six hens then two of each breed, or all different breeds may save you some social problems.
Try and look for breeds that are known to be breed tolerant.
Try and avoid the situation where you have more than one of a particular breed in an otherwise mixed flock.
Finally beat in mind that having hens of similar body weight can help to avoid bullying.