I have a diverse group of brassicas growing this year which also flowered and set seed with overlap, and made me wonder which ones could cross with each other, it is super confusing. At first I was searching for species names thinking this would be a barrier to crossing but it may not be as good as I thought! See the chart below which showed the rate of successful crosses when different groups were hand pollinated.
Lacinato kale, collards and broccoli are in the same B oleracea which is relatively secure from crossing with the others. But the mustards (B juncea) and oriental cabbage, bok choy types B rapa, and Red Russian Kale B napus . are apparently easier to cross than I thought.
I've saved all the seed anyway because all of them make a fine baby greens to cut and come again in the winter. Some of the original seed planted this year was labeled 'random brassicas' and came from my mom's 'everything brassica' patch. There are several types not seen in the original, and they are good eatin.
I'm planning to start some seed soon, just to see what comes of the different packets...