My only thought on the plum is that there might be a chance wild plums might pollinate your plum, though if it is a Japanese-type (round fruit?) then that is not likely. My 'Enterpise' was and is
being pollinated by a volunteer crabapple that a kind bird started for me about 30 feet away as I didn't know about needing a 2nd cross pollinating variety when I first planted it.
I've planted 'Pristine' and 'Ark Black' near each other to cross pollinate. The fact that they do this (I.e. flower together) and grow harvestable apples in July and October still astonishes me.
If you had not considered disease resistance, and you've bought them already, you might want to review their resistance profile now. (Usually, Google search on the variety name and "disease resistance" yields reliable information) In my area, Eastern Red Cedar and Atlantic White Cedar are common regional plant community species, so Cedar Apple Rust resistance is VITAL. Also, the many subdivisions, office parks, and town developments in this area date back to the 80's when Bradford Pears were introduced and flooded the landscaping business, so we're inundated with the pesky trees, not only in intentional landscapes, but those that have escaped cultivation. They provide the vector for Fireblight.
That and the clay subsoil (limits some root stock selection) calls for very "interesting" care in choosing apple and pear varieties.