Combo Disease-Resistant Apple (4 varieties)

Only one plant needed? Or do I need 2 or more to pollinate?

Due to having several compatible variaties grafted on the tree it is "self fertile" but will not be if enough variaties are lost.

Are these trees mature enough on arrival to companion plant around or is it better to wait a certain number of years until they are more established?

These trees are 3 years old when shipped. Depending on what you are planting it should be fine and is preferred so you don't damage the root system by digging around in the root zone later.

There's a lot of junipers where I live, is this resistant to cedar apple rust?

Yes. These apples are all rated resistant to very resistant to cedar apple rust.

I was wondering how many years does it take to fruit after planting this?

Typically within 2-3 years if the tree establishes correctly.

Is this scab resistant?

Not specifically no, it will need regular pruning and good light penetration.