1) You should really only breed 2 star boars. Make that a priority. You only need one , you can get a 2 star by breeding with a no star. Once you have 2 eliminate anything that isn't 2 star.

2) They need to be touching in order to "mate" (sounds obvious). The best way to accomplish this with boars is to let them all get hungry ( they cannot die or untame by starvation). Then toss a stack in the middle most point.

3) Space, the more the better. Population growth is limited by available space.