Would the damage dealt to multiplex be reduced by 1 for each adjacent multiplex?

That would be cool, but no. It doesn't matter how many Multiplex figs are adjacent, so long as there is at least one other they only gain Toughness.

Lots of wasted potential on this fig.

Nope. It's a yes or no. Is he adjacent to a Multiplex? Then reduce by 1. If not, don't reduce.

Just to be perfectly clear Multiplex doesn't really get Toughness - he just gets to reduce damage by 1. Same effect, but not quite the same thing - for instance Mutliplex cannot be assigned feats that have Toughness as a prerequisite (Shellhead is the only one that comes to mind...)

it would be awesome to be able to reduce by 8