As for CDL, as long as the truck is under CDL (26k) and not a primary use as tow vehicle (set up like a tractor with no bed), you shouldn't have to worry about a CDL. That said, ask 10 different DOT/Transport Police people and you'll likely get 11 different answers...
While I definitely agree there are a lot of gray areas in the DOT rules and you can get lots of different answers one thing that is very clear in the books and with every officer I have ever encountered (I have a DOT number and 5 trucks) is that they add the GVWR of the truck and the GVWR of the trailer to determine your status. If the two of them together add up to over 26k you need a CDL and everything that goes with it like logs books, commercial inspections, medical cards, DOT number, etc.
A lot of trailers have a GVWR of 14k so you want to make sure your truck is 12k GVWR or less if you are going to pull these trailers or else you can get into a big mess. I have the tickets to prove it.
In order to determine if you need a CDL license, you need, you need to understand how the Gross Vehichle Weight Rating (GVWR) is determined. First, the Gross Vehicle Weight Rating (GVWR) for each vehicle is established by the truck and trailer manufacturers for their equipment. For licensing purposes, your combined GVWR is determined by adding the Manufacturer's Gross Vehicle Weight Rating of your Truck plus the Manufacturer's Gross Vehicle Weight Rating of your trailer.
When do I need a CDL?
A Class A CDL License is required when the combination of the Gross Vehicle Weight Rating (GVWR) totals 26,001 or more provided the GVWR of the vehicle(s) being towed is in excess of 10,000 pounds.
A Class B CDL License is required for any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing a vehicle not in excess of 10,000 pounds GVWR.
A Class C CDL License is required for any single vehicle, or combination of vehicles, that does not meet the definition of Class A or Class B, but is either designed to transport 16 or more passengers, including the driver, or is placarded for hazardous materials.
One of the most misunderstood parts of the above requirements has to do with trailers with a GVWR in excess of 10,000 pounds when the combined GVWR does not exceed 26,000 pounds. In this case a CDL is NOT required provided you are not placarded for hazardous materials. However individual states may impose a requirement that drivers in their states must have a CDL in this situation so you need to check the requirements in your state. It is also important to note that individual state CDL licensing requirements CAN NOT be applied to drivers from other states.