I have leeks, shallots, garden chives, garlic chives, and green onions planted in my garden. It is possible they could cross, but it hasn't happened yet. Green onions don't bloom until their second year, and I pull my leeks before they bloom. The chives don't always bloom at the same time.
Plants can only pollinate with their close relatives not with members of different families or family members that have different number of chromosomes.
If you are saving seeds from the onions, it might be better to isolate them from each other, or not collect seeds if two species are blooming at the same time and may have crossed. Otherwise, it really does not matter.
Onions are great companion plants and repel a lot of insects from other plants.
But they should not be planted near beans and peas. They actually attract the same pest, black aphids.