Your buns and foster buns

I would love to start fostering some rabbits next year. Im a bit worried about my bunny Billy who is very territorial and whenever another rabbit has been in the garden he takes his aggression out on poor little Gizmo. My buns are free range and Billy kind of considers the whole garden to be his. If I have foster buns in separate enclosures, will Billy ever get over his referred aggression or is it likely to always be a problem?

I find that keeping a few feet of space between pens/hutches, and creating a barrier so that the rabbits can't see each other works well.