Plum Grafting and Base Tree

I am planning on planting two plum trees, where each one would be multi-grafted. Because I am in Colorado, the majority of the plums I would be growing are E plums, including Empress, Castleton, Green Gage. There are a few Asia plums we would like to grow, but they will be more of an experiment than a staple, since E plums do much better here. Another one we would like to add to the mix is Bubblegum/Toka, as they do well here, though I think it is a Asia/American hybrid if I understand correctly.

As I see in the CRFG chart, you can put many Asia plums on an E plum, but generally not the other way around (for actual wood compatibility, not pollination).

Am I best off doing 2 E-plums as the base, versus one E plum and 1 Asia Plum?
Can Bubblegum/Toka be grafted to either?
What makes for a great base tree? Mt Royal would be a strong performer and hardy for our climate, though not the first choice for fruit

Iā€™d use prunus americana as a base and then go from there. Just my own preference as I use it for most all of my plums. Toka is a hybrid p japonica x p americana


Does anyone know what Toka/Superior can be grafted on-to, and what can be grafted onto Toka/Superior (E vs J Plums)?

Prunus cerasifera although it may be subject to brown line disease when grafted to European plums.