.....I've had the same experience this year and last but with the exception of a mojave. I have removed atrox,mitchelli, tigris and mollosus from the same property within very short periods of each other. I attribute this to the fact that this property was irrigated and it was during a period of extended drought for the valley, which was the norm for most of the season.
I have also seen three of the above mentioned species within the same small rock pile and never come across anything resembling a hybrid. With the amount of time I have spent in that range, if they were remotely common, I might have come across one by now.