I grafted a santa rosa plum onto a reliance peach. took pretty well with the cleft graft that I used. I'm planning on putting more stone fruit on that peach.

-Little John

I'm no expert, and have minimal experience with grafting stonefruits, but get the impression that there's quite a bit of latitude in what will and won't work. I would suspect that the pluot/aprium combo would be no problem at all.
Peach seedlings seem to work well as an understock for many plums, apricots, etc.
I've used Nanking cherry as a dwarfing understock for peach and Japanese hybrid plums, with reasonable success. It was an experiment, and those trees are long gone now, but lived for a number of years before borers took 'em out.