What varieties of trees are compatible for grafting with cherries other than other cherries? Plum, what else?
IIRC, anything from the "Prunus" family, which includes plums, apricots, peaches, nectarines, and almonds as well as cherries.
My personal rootstock preference is for "Mazzard" but that may depend on your soil type.
The link I revised the Mazzard mention to point to does note some specific cultivars having issues with specific rootstocks other than Mazzard, which probably means that some other in-family members may not actually work with at least some cultivars. But generally, they all at least could work.