It's a lot simpler then people are making it. Take the weight the trailer is rated for 26000- then minus what the trailer weights guessing about 7,000, that is what the trailer will haul. Then make sure your truck is capable of towing that, and the pin weight needed. The 5500 will handle that no problem. I've hauled 18,000lb mini ex on my gooseneck with 3500 srw, it tows fine. In many cases these goosenecks are incredibly overbuilt. Many of the 2-10ks are exactly the same as 3-10ks.
DOT varies, but generally as long as you are under axle and tire weights, you're fine.
One FYI 12k axles are very expensive to buy and maintain compared to 10k's. Depending on use, I would consider going to 3 10k's. Cheaper to buy, and would cost about same to maintain if you paid someone factoring extra labour for extra axle. But gives you a lot more capacity, and considering the 5500 will tow 30k, gives you more room if you need to haul something heavier.