Welcome from Gainesville!
Regarding underquilts, there are multiple cottage vendors out there who offer high-quality down products. Take a look at what style of hiking you tend to do and how you sleep. That'll help determine whether you want a 2/3, 3/4, or full length underquilt and what temperature rating you want.
As to the tarp, you got the stock Hennessy diamond, yes? If that's the case, then, yes, it is most definitely small. However, if used properly, it will keep you dry in just about anything (most famously, Sgt. Rock here on the forums managed to stay dry through an hurricane using one). However, it's worth learning how to use it as well as possible; broadside to the wind and slightly down the slope in a draw if possible. Practice, practice, practice is the key here. Or, if you're willing to spend on a larger tarp, well...
For a good overview of tarps, underquilts, hammocks, and all things hammockry related, it would definitely be worth coming out to the Third Annual FL Hang at Buck Lake in Ocala National Forest over the weekend of 1/18-22/'13 (that's MLK weekend). The links for it are in my signature, below.
Welcome to the madness!