Hi, they can cross-pollinate. Here is an article about different kinds of Marigolds: The Marigold: Easy-to-grow, Colorful, a Timeless Beauty! The hybrid T. erecta x T. patula is supposed to be sterile, but I've grown a few marigolds, I believe, from hybrid's seeds and they reverted to T. erecta (at least those few marigolds).

I have been working with the same bunch of different color marigolds now for 4 years and this year I have noticed some crossbred flowers coming on and they ar beautiful to say the least i started with some solid orange yellow along with red with orange tips! this year I did as i always do collect the seeds and mix them all together usually results in the same color blooms On the plant however this year was a different story many of them are completely different colt like a swirling the petals see picture! Good luck if this is the outcome your looking for and this is all on the same plant! Absolutely amazing!