Yes, I have tried using a lycra skin with my semi-dry and it did break the seals. The semi-dry turned into a normal wet-suit.
As for getting in and out of an exact fitting heavy 7mm it is all a question of developing an efficient technique.
This is a from a post I wrote back in 2009. I still use exactly this technique:
If this is a well fitting 7mm semi dry then there are definitely some tricks to using it otherwise you'll be too tired to dive after the work of donning it.
The first trick is to turn it almost inside out leaving about 6" at the ends of the legs and arms folded over. Then I use 2 smallish strong plastic bags. I put one on each foot and push them into the leg openings pulling the fold. You will then be able to just roll the suit up your legs and body with an absolute minimum of effort until you get to the height of the base of your sternum.
Transfer the 2 bags to your hands and push them through the still folded arm openings. Then just stick out both arms and the top part of the suit will roll into place. Finally head into the collar and work it down with both hands. If I'm not wet I do sometimes use the plastic bag over the top of my head - always guaranteed to produce a few smiles on the dive boat - but it really extends the life of my latex neck seal.
The combination of the strategic use of plastic bags and the rolling techniques reduces the effort to a fraction of what it would be.
You'll need a buddy anyway to close (and open) your zipper if it's the rear entry dry-suit type.
Assuming you have a buddy to unzip you, the real trick to taking it off once you get your head out of the upper part is to work the suit down a few inches around your torso before folding the suit over at the shoulders and then rolling it off.