I also dive with a 7mm Farmer John style wet suit, and I do dive with a skin underneath it. I used to use a fleece lined Polartec, but just wasn't completely happy with it.
I changed to a Sharkskin a few years back, which is basically a thin plastic suit (I believe the fabric is polyolefin), and am very pleased with it. Aeroskin is another company that makes a similar suit.
They do make the thicker neoprene wetsuits slide on and off easier. The polyolefin fabric also sheds water very quickly, and dries in a fraction of the time that it took the polartec to dry. If you're doing multiple dives, it makes it easier to put the wet exposure suits back on after your surface intervals.
I also wear the Sharkskin under my shorty when I'm in warmer water. If I'm making 4 or 5 dives a day, it keeps me from getting chilled toward the end of the day. It can also give you a little protection from any accidental contact with things that could sting.
Hope this helps.