i think the rawl bolts are normally 8mm for metpost bolt ons, but have some washers handy as well.
As for adapting the panels, try to do it at the end of a run, or where it will be obvious. As you have siad remove the battens from the end, and place and secure them to the new edge of the fence. I normally use a battery trim saw or a decent sharp andsaw to cut them. Lay them down so they are well supported when you cut them as they do flex a bit and can be a sod to cut if they are damp.
As for adapting the panels, try to do it at the end of a run, or where it will be obvious. As you have siad remove the battens from the end, and place and secure them to the new edge of the fence. I normally use a battery trim saw or a decent sharp andsaw to cut them. Lay them down so they are well supported when you cut them as they do flex a bit and can be a sod to cut if they are damp.