Anyone  have an opinion on the best rig for skipping soft plastics under docks and tree limbs? Casting or spinning? Any particular model reel, length and stiffness of rod?

I have a 6' 8" M/F "Wacky Style" Mojo with a 35 size Pfluegar Supreme XT that skips baits very well and has enough backbone to pull them back out of the cover. Most any spinning rod will skip pretty well with the right lure and line though. 

Skipping  with a caster is a worthwhile skill to develop but if skipping will be the primary presentation spinning is just so much easier and effective. An easy loading fast action Rod produces speed to help a bait skip well. A longer Rod 7' or so generates more tip speed but a shorter Rod may be more convenient in tight places. 

The Daiwa SV105 baitcasting reel is very easy to skip with.

I use a shimano citica201hg on a medium action rod for soft plastics. Never had a problem using a medium action rod when it comes to pulling big fish out of cover and it lets me load the bait for more distance.

I can skip with both casting and spinning gear, spinning is a lot easier.................esp. with weightless stuff. I tend to skip jigs/frogs/etc...with the casting gear, and use the spinning gear for yum dingers, or other "light" baits. As for rod length, at least with spinning gear, I used to use a short 6' rod, and it was super accurate with it, but it served me little to no purpose out side of skipping. I tried a 7' rod, and although that rod has more uses out side of skipping, I didn't care for it. I now skip with a 6'8" M/XF...........best of both worlds IMHO. For casting rods, any length over 6'6" but under 7'6" do just fine for me, with a 7' and 7'3" the "sweet spots" 

If you're looking to do it occasionally, you can do it with casting gear, but if you're looking to do nothing but skipping, a spinning rod will be much easier in the long run. Not to say you can't get really good with a bait caster, but it takes much longer to learn.