Alright, subtract the weight of all the passengers and stuff you plan on carrying in the truck from 1592lbs (average for a 1/2 ton truck). Subtract another ~200lbs for the weight of the hitch in the bed as well. What you have left is what you can have for the fifth wheels pin weight. If pin weight is under that remaining weight then you are good to go. Keep an eye on your rear axle weight rating too. Being a little over payload isn't that big of a deal as long as you're under the RAWR. That's a little harder to determine without going to the scales.
You will be able to tow a much larger trailer than 5th wheel though. With a fiver the truck needs to carry the pin weight which is 20-25% of the trailers weight. With a tow behind trailer you have the tongue weight but it is much lower, usually only 10-15% of the trailer weight.