If your clay soil is as bad as mine it will probably dwarf any tree. I would get a standard tree and hope it grows as good as a dwarf would in normal soil.

There's nothing you can buy that will work as well alone as mixed with at least 1/3 mix with your clay soil. Suck it up and take a pick to the clay under your beds and go down at least one foot. Clay has lots of nutrients, the roots just have trouble penetrating it. Mound up your bed to allow for future settlement of the mix.

I have a raised bed orchard. I put 3 varieties of pear, peach, or plum in a bed. When I started I took my spading fork and sunk it as far into the soil as I could get it and rocked it back and forth to loosen the clay. Then I added 3 inches of compost 3 inches of soil conditioner and 6 inches of garden soil and let that settle for 3 months or so before planting. The compost will draw worms which will help to areate the compacted clay soil. I grow all dwarf varieties as they provide more than enough fruit and are easier to keep in a manageable size.