How big is the space? Your talk of sticking them all in one hole makes me wonder if you are quite aware of the size of these trees?

I wouldn't worry about getting the wrong pollen, the bees will sort that out. They'll visit all the flowers so they're all bound to get compatible pollen. Kirke and Coes will pollinate each other. And the pollinators can cover quite long distances. The trees don't have to be crammed together. They could be on neighbouring properties and they'd still be close enough.

Since the plot has 4 cultivars they should have been chosen to have an overlapping flowering period.

But I would worry about the trees having sufficient space.

Pollination partners

You could look to plant the two Euro plums in one hole, but i wouldn't recommend all three. When you plant in one hole, you want to prune to keep between the trees open and that could cause you to kill off one variety of the Pluots. The Pluot tree should be pruned to keep all of the varieties at as close to the same size as possible, so couple that with pruning 3 trees in one hole to keep and open middle and you are just going to give yourself a lot more work than needed.

Being in Zone 5a, i would strongly recommend as much protection from the north and west winds for that Pluot tree as it is borderline hardy for Zone 5 (im in zone 5b and have been too scared to try pluots).

If you keep the plants within 50 yards of each other, you can be assured of getting solid pollination from the insects.

I'm saving that super sunny spot on the south side of my house for zone tender trees. I've got some nectarines there now, for example. I do have an existing European plum "orchard" (only three trees, Italian, greengage and Mirabelle) nicely situated up on a bit of a southwestern slope. Great sun and drainage. Could put the pluot in the protected spot, and move Coe and Kirke up there. Should be enough room, but I'm not sure if Kirke and Coe can stand the cold and wind of 5a. Are they as hardy as the other Europeans? I hear they can be fussy.

I'm not seeing these offered by anyone other than RainTree in the US. I have never ordered from them so i don't know how their zone ratings stand up (by that i mean i have seen some nurseries say Zone 5 rated to -10F or -15F (Gurneys is one of these). That is a nice mild winter here in 5b if we don't drop below -10F).

OrangePippin has a tree register and i took a quick look there, but i am not seeing anyone growing these in a zone 5 climate. To be honest i had not hear of either variety before, but that Coe's has me interested (if only i had a bigger yard!). If you have the space, I would say give them some protection, the Pluots definitely need it though as a bad winter could kill that easier.

Coe's Golden Drop is a Greengage and you say you already have a Greengage. How does it fare?

It seems well, but it has only been in the ground 1 year...