5 toon just works nicely with main toon being center, 2 go left and 2 go right. With 8 it will not be so neat.

If you look at the mapped key for line formation you see it is actually made for 6 non-active slots, i.e. total 7 toons including the active slot.

The target of the movement key is (@other isboxer&~dxnothing)#1 which means "from the set of loaded slots except the current and except dxnothing, select the first slot".

So for 8 toons, all you should need to do is add an extra action to also move the 7th non-active toon: (@other isboxer&~dxnothing)#7