Yes, there are many species of apples. And most of them cross polinate. End of that discussion?

If OP wants to grow small apple tree bonsai it may make sense to grow small fruit trees no matter if it is called crabapple, malus sp or anything else. In any way it does not make sense to me to grow from seed. You will not know size of the fruits in many years. And maybe one day it turns out that it is growing huge fruits. Better to start with a tree you have seen the fruits on.

The tree in this video has grown from a compost pile close to my forrest. There are absolutelly no wild apple trees around, so it is most likelly that it grow from an eaten apple. Fruits is less than 10mm on this tree as well:



This is how many seed sown apples turn out, you just don't know before they start to bear fruits.

All dessert and cooking apples are grafted onto specific rootstock, many developed at Malling in Kent. The rootstock used depends on how big the tree will be from dwarf up.

Would be correct if you said most, not all. As I informed in a previous post my family have an apple orchard where they graft on wildly sead sown stock. That is a traditional method on rocky soils in Sweden and Norway.

The apple tree on my photo above is also grafted. I have no idea why really. But the graft is looking really great so I don't mind.

For bonsai I don't think grafted dessert or cooking apples is an option. As been mentioned the fruit itself don't reduce in size like the leafs do. For bonsai it is better to go for varieties with smaller fruits.