Is there any truth in that the heirloom plants cease to be heirloom when they get pollinated by something that also pollinated a hybrid plant?
Dude that is a great question, I bet it would cease to be a heirloom at that point. I will ask the master gardener in my life to see if she knows.
I do know that you can pollinate plants yourself and not worry about it if you are really worried about it. I learned how the other day. Basically you take the male "flower" from the plant and rub it on the female "flower" of the same said plant then you tape up the female "flowers" petals and keep pollinators out. I know this works for things like melons and squash, I bet you could even do it with corn and just put plastic wrap over the silks of the ear.
interesting little fact did you know the for every strand of silk (female flower) there is a possible kernel of corn? If some of your kernels are not there that means the pollen from the tassel (male flower) did not hit that particular strand of sil on that ear of corn.
If you want to keep an heirloom and also play with some hybrids, you can plant both and detassle the hybrid. Your heirloom will then pollinate itself and cross with the hybrid. Thus you get a plant that is predictable and one that is a little more custom taylored.