Hi Peter.

often from the short line out you aim to hit a weak part of the defensive line. Often you see the 10 channel get hit or aiming at a weak defender. So you want your forwards in 2 pods, one targeting the hit area and the other to set up next phase or support next phase. 

They can between 9 and 10 but as long as they are out the way of the passing line.

Remember if your lineout fails oyu still need forwards close by to react.

In the diagram (from a 3 man line out) (forwards numbers don't relate to position) forward 5 can come in to help if the ball is spilt from the line out etc. 5 and 6 offer running lines either side of 10 or if 10 hits up can ruck over.

6 and 7 offer runners for 12 or if 12 hits up ruck over.

7 and 8 offer runners for 13 or if 13 hits up ruck over.

You don't want to go any wider than that.

I usually have a play where 6 comes from deep support to take the pop, The back. 5 and 7 ruck, Then you have options to go same way with 8 as a support and the line-out players coming round also or go back inside towar the line out. Keeps teh defence on their toes