The weights are easy to find on a chart/owner's manual and are generally good guides. Also consider the height of the trailer, at high speed the wind resistance may be so much that the weight won't be the limiting factor. As for a tranny cooler, it's not always needed nor advisable. Most tranny heat is generated in the torque converter and your truck has a locking converter; once it's locked, no more heat. If this trailer is a toy hauler that'll be pulled off road--low speed hilly terrain then you may have heat issues. In that case you may even need a cooler with a fan. Adding a cooler without need may keep the fluid too cold-also not good. In any case, step one is a tranny temp gauge! Even if you don't think you'll need it, get it and pay attention to it! You may find you never need a cooler. I regularly tow much above my Ex's rating and have never seen above 170deg. and on the highway keeps 150ish, but I'm very careful about it. If you find that you're getting into the 200deg. area at any time STOP, but don't turn off the engine! Run it in neutral/park at about 2000 rpm 'till it cools then continue. If you get hot in your normal use of the trailer get a cooler and consider one with a fan if the heat happens when driving really slow speeds as the stock fan may not pull enough air under certain conditions to make full use of the cooler. Also install the cooler in the line before the stock radiator cooler to help minimize over-cooling the fluid.
