Lots of experience with myro and it seems compatible with all plums and pluots. Pluots are compatible with anything plums are- they are essentially J. plums with no discernible apricot traits I’ve ever observed in any I’ve tasted or grown. However, my experience of grafting pluots is only on Myro, because that is my go-to plum rootstock here. I know that Flavor Grenade is not especially vigorous and I would prefer that on more vigorous rootstock, but Wilson seems to always put it on Citation, which is not vigorous enough for my needs with FG.
I have successfully grafted J. plums on E. plums and also the other way around, no matter what the literature says (which is that you shouldn’t graft E’s on J’s. but you can graft J.s on E’s which is what you want to do anyway). Haven’t been doing it long enough to know if the greater vigor of J. plums to E’s becomes a problem if you are trying to balance both in the same tree.