Rose to Rose, Plum to..... almond: Inter-Genus and Inter-Family Pollination in Fruiting Trees.

Latin naming goes: Family > Subfamily/Tribe > Genus > Species > Cultivar

When it comes to pollinating for setting fruit, remember:
1) GENUS can always pollinate GENUS, and SUBFAMILY can usually pollinate SUBFAMILY
This works fine if your goal is setting fruit, not producing viable seeds.

If you want viable seeds from these fruit, and you're trying to get a next-generation via seed, then Species-Species breeding is better.
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Most popular fruiting trees are all in the 'Rose' Family, and there's always a small chance that all of them could, in theory, pollinate each other.
I knew a guy who deliberately pollinated his blackberry canes using rose pollen as an experiment, and it worked just fine. It set fruit as normal, but the seeds gathered from those fruit didn't sprout. Incompatible hybrid.

All Prunus can pollinate each other.  (All cherry, almond, peach, plum, apricot, etc. can pollinate each other)
All Rubus can pollinate each other (All Blackberry, raspberry, dewberry, etc.) <--boysenberry and loganberry are hybrids!
All Maleae can pollinate each other (All types of apples, pears, crabapples, serviceberry, hawthorn, quince, loquoat, etc.) <-- some inter-genus hybrids in this tribe are even seed-viable!
All Citrus can pollinate each other. It's not a 'Rosaceae' family fruit, but it's worth mentioning that Citrus is freakishly good at hybridizing with other citrus. (Lime, Pomelo, Lemon, Orange, Kumquat, Grapefruit, etc.)

Bloom TIME is the biggest issue for natural (not human-assisted) cross-pollination.
If you have two cultivars of pear but they bloom at entirely different times in spring, you could add an ornamental crabapple that blooms all spring and use that to pollinate both pear cultivars.  

If you planted a BUNCH of the same early-blooming cherry cultivar and they won't cross-pollinate properly because the clones can't self-pollinate, adding an ornamental Flowering Almond that also blooms in early spring will pollinate them.

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One of these days I really ought to make a graph of different Rose Family plants, which ones bloom at what times, and which have a proven track record of good pollination with the other.
All the lists I see are focused on JUST peach cultivars pollinating other peach cultivars, or apple cultivars pollinating other apple cultivars.

Lemme know if you're interested.

Okay, now that's interesting! I knew the Rosaceae have notoriously weak crossing barriers, but hadn't considered that would mean there's a possibility of fruit set even in the incompatible crosses... Cool!