I don't have too much time right now, so this will be a quick answer.
Planting distance: Depends on root stock your apple is grafted on.
Try here: https://www.brogdale.org/chooserootstock.html
I thought beause apple and citrus trees give crop at the winter so it will acceptable to be also near.
LOL
That would be convenient for YOU, I'm sure. But you have to think from the tree's point of view and what IT needs.
I didn't realize you're in Greece, but you are mistaken if you think North and South sides of your house, garden, whatever are the same.
Look up "microclimate". Even whether the tree stands alone in a field of grass/vegetation, surrounded by hard (flagstones, concrete, etc.) surface, or a bare mulched (light colored or dark colored) circle makes a difference. The slope of the land. Wind-break in the form of other trees/shrubs/building. A nearby heat/light reflecting structure/wall. They all make a difference in the immediate environment of the tree.
Finally, it's generally considered a good idea to separate crops that suffer from same predatory insects and plant a different crop in between them to confuse the buggers. My personal preference is to use no chemicals and invite beneficial wildlife -- birds and insects -- to come dine on the harmful insects.
Look in Organic Gardening, Permaculture, and Wildlife Gardening forums for discussions about this topic. Look in Permaculture for "apple guild".
