If you could only take one rod to the Guad to fish for trout, what would it be? What length, weight, action, brand, model?

Personally, I don't think that the rod makes a huge difference when it comes to trout fishing on the Guad when you're fishing nymph rigs. Although nymph rigs are a pain to cast with indicator, split, and two or three flies, you're not making a long technical cast.

I do think a longer rod helps with mending and controlling line so a 9' would be preferred over an 8', but the type of line you pick is probably just as important. Something with some weight in the back belly helps with mending and roll casting. I'm currently in love with Scientific Anglers' infinity taper. It really seems to do everything well, big flies, roll casting, distance, accuracy; but I haven't gotten a chance to get in some real current and see how it mends.

The rod I have used at the Guad most often is an older custom Sage rod, a 596-3 SP. 9.5ft, three piece, moderate action (I think Sage said that SP stood for Smooth Performance). I like the reach and sensitivity of the rod, it has enough backbone to handle heavier rigs and the occasional heavier trout, and it is a joy to cast (well, if any rod is a joy to cast when you are using double nymph rigs with weight under an indicator(.

I dont think it matters as far as brand and model, meaning I dont have certain rods for certain rivers, with that said I dont even have certain rods for certain species. Action is more about your comfort in casting. Yes I have various weights and lengths and different lines for different techniques. If you dont have a rod yet, I would start with a 9 ft. 5wt., it will handle anything on the guadalupe. WF line is very versatile, whether for nymphing or throwing streamers. If your going to throw double nymph, tie your second through the eye of the first, not the hook. Also use foam indicators, two if you need them instead of the plastic round indicators. Easier to cast!
Good Luck!