Crossing occurs readily with like species ie: P. rhoeas and another P. rhoeas, but P. rhoeas will not cross so easily with P. somniferum or P. orientalis. What I'm saying is that the subspecies within the genus papaver will not easily cross, but if you want to prevent a pink and red flanders poppy (p. rhoeas) or a purple and red P. somniferum from crossing you wouldn't want them planted together.
If you are worried about labeling them for trades just call them or inform the trader that they are open-pollinated poppies. That's what I did and I'm excited to see what colors I get next time around. I don't think anyone I've heard here (or elsewhere) keeps them separated for fear of crossing. You still will do doubt get some that are not crossed each year, but also have some that have. That's the beauty of it :D

Vera

Vera, Thank you for the clarification.
Gloria