Keren, the only case where you want to amend the soil is if you dig a hole and fill it with water and it takes more than a minute to drain. In other words, heavy clay is OK as long as its not super heavy. If your clay does not drain then you may need to amend; I can't help you much with that since my clay drains OK. Amending if you do not need to can be a really bad idea, it can restrict how far the roots travel and make your trees root-bound.

In terms of placement, the trees besides the cherries will prefer full sun. Don't do afternoon-only sun, that is never good since the dew will stay on a long time and diseases will blossom.

The red leaf plum should pollinate the Friar so I think you will be OK there. The red-leaf plums are crosses that usually have some Japanese plum in them.

Scott

Soil amending can mean more than one thing. In a heavy clay, even one that drains adequately, you can accelerate establishment by using about up to a half yard of black compost or humus and mixing it thoroughly with the top few inches of soil in a six foot radius, creating a slightly raised bed for each tree. Spread some of the roots high and they will flourish in this better aerated, more quickly warming environment

Keep the top soil on top when you plant. It is pouring amendments down a hole that can create problems.

If you are going to the trouble of defending plums for fruit I'd suggest a tastier variety than what I suspect your
All Red to be. EL charges a lot extra for shipping their trees in pots anyway, so you get much less tree for the money.

Yikes. I had planned to dig big (and deep) holes and mix in about 1/3 peat moss. Sounds like that's a recipe for disaster... but... I don't think my soil drains all that well, so, what do you recommend? I'm planting these in my yard, so a six-foot radius for each tree is a LOT of grass that has to come up.
Bummer about the Red Leaf Plum but at least it will pollinate my Ruby Queen, which should be better tasting. One pretty tree, one tasty one...

The only reliable ways of dealing with poorly draining soil is either installing drainage, if there's enough slope to drain water away, or planting on raised beds or mounds.

Plums are particularly adaptable to heavy clay, so if it's good enough soil for most other trees to grow in your trees should be OK. Using the method I described would be helpful regardless, and the grass will grow back and help hold the raised soil.

Keren, I would do the drainage check I mentioned above before deciding on your plan. Poor drainage is a highly relative term.

Scott

Peat moss holds moisture a long time, so it probably isn't the best thing to add to clay.