I'm even wondering if one of the native American plums might be best for a root stock, like P. Americana or P. maritima.

Apricot would me my choice also...not suckering like a wild American or Beach plum.

you'll have a hard time with apricots in maryland. you don't have to have different fruits either. Even a peach tree with three types of peach is great as you can spread out the season by grafting peaches that ripen early, mid, and late.

Tomcot supposedly does well here for an apricot.

What about a "commercial" rootstock like Citation? It's used for peaches, plums, and apricots, could I let a Citation grow out to a size where I can train several limbs to graft, instead of budding a single cultivar down low like you normally would?

I have 3 cocktail trees and I wouldn't say they are rife with issues. I do have to stay on top of pruning and another tree at my Dads lost 2 of its 5 grafts because they weren't as cruel with pruning as they should have been. I see them as a good candidate for 2 reasons. One, being able to fit more than one fruit variety in a single space. Two, being able to choose varieties to stagger the harvest times so you have fresh fruit over a longer period as melikeplants alluded to.