I've got LF Orps, 3 are very happily mixing with 4 Black Rock, another is in with the offpspring of Warren (are they 'Warren' too
) and another 3 are happily mixing with several Light Sussex. Can't say I've ever had a problem with them. The Black Rock are a nightmare and want to fight everything that moves (other than the Orps that they know) but otherwise everyone gets on well.
The biggest problems I've had with regards to mixing have been when introducing new birds to an established flock, irrelevant of which breed of chook.