I use an underquilt with mine, but still have the double layer - it can be convenient for several reasons. One is that you can always use the double layers for extra insulation, or switch back to a pad if you are afraid that you might end up having to go to ground due to a lack of trees. The big benefit of a double layer, though, is when it's warm enough that you don't want bottom insulation. Mosquitoes can bite through a single layer of ripstop fabric, but two layers blocks them. Very handy.
I believe Brandon sells carabiners separately for the webbing suspension. Or you can use any two climbing rated carabiners. Or, even better, Jacks-r-better (
http://www.jacksrbetter.com ) sells the Dutch Clip, which is specifically designed for hammock suspensions and weighs about half as much as a set of carabiners.
I guess a tarp COULD be too big - if it's too long to fit between the trees or too heavy to carry, but otherwise you're golden. I actually own several tarps that my fiance and I use with various configurations. We each have a 12x10 tarp in the same design as a Speer Winter tarp, which can be closed up to give all around coverage for cold weather or privacy. We each also have some small tarps, for times when the weather is really nice and we want more of a view out of the hammock.
The Superfly is a great choice because it combines both of these options into one tarp. With the doors attached, you have a huge, spacious wrap-around shelter that will provide privacy and a wind block. Pull the doors off, and the tarp is still very big, with plenty of coverage for serious rain storms, but lighter and can be pitched higher to provide more air flow and a better view.
For us, the fact that we often hang together under a single large tarp made the modular superfly not the best choice, but if I were going to be hanging solo more often, I would definitely consider a Superfly.
