If planted in-ground don't add anything to the soil placed around the roots. Just the straight soil taken out of the hole.

If in a pot there are better media for an apple. The 5-1-1 mix from the containers forum would be better.

SPM is very acidic and best used for potted blueberries.

I think that I have 90% clay soil. I heard its recommended to use peat moss or some sort.

In clay soil amendments aren't usually recommended. Some say it can result in a bathtub effect and drown the trees. If your clay drains well it won't need amending. If it doesn't drain well I'd recommend planting in a raised bed at least 12 inches above grade.