I can't see a 0.5mm layer making any significant positive difference in your warmth under a 7 mm suit -- less than 10% more insulation. Only issue I can immediately think of would be if your 7mm fit so poorly that it was pumping water all over you.
Adding another layer means twice as much Nylon to dry, as well as the possibility of trapped water. You may end up very chilled between dives unless you strip out of the suit and dry off completely. No need to start your second dive already cold!
Remember that wetsuit manufacturers routinely claim that 2.5mm is warmer than 3mm, and that spraying titanuim magically keeps you warmer like a space blanket. Considering that a space blanket deals with reflecting radiant energy and the overwhelmingly dominant heat loss in the water is conduction, would you trust all the advertising? (Though there is good reason to believe that the smooth-inside suits may help in reducing the amount of water that sloshes through you, old "Nylon one" suits were always warmer than "Nylon two" -- funny how it all comes around again.)
IMHO, take the $150 that the skin would cost you and buy a custom 7mm that fits you right instead of an off-the-rack one that you need to fill with a skin...
...or a drysuit!
