Here is some info from the FMCSA site. You can read all of it right here
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Classes of License:
The Federal standard requires States to issue a CDL to drivers according to the following license classifications:
Class A -- Any combination of vehicles with a GVWR of 26,001 or more pounds provided the GVWR of the vehicle(s) being towed is in excess of 10,000 pounds.
Class B -- 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.
Class C -- 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.
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This means that you don't have to be driving a truck with as GVWR over 26,000lbs in order to get a "Class A" CDL. Note also that you can't get a "Class B" with you F450. So any truck and trailer, regardless of size, which puts the combination over 26,000lbs GCWR would work for the CDL road test.
After re-reading your posts I realized you said your traier was RATED at 20,000lbs and REGISTERED for 14,000lbs. The registered weight is a tax issue and separate from the rated weight - which applies to whether or not you need a CDL. You will definately need a CDL in order to tow your trailer legally since it is rated at 20K. Hook up to any 3/4 ton pickup or larger and you will be over 26,000 combined rated weight. It doesn't matter what the truck and trailer actually weigh. This is where most people get confused. You need find the actual weight of the truck and trailer you will be driving and use this number to obtain your registered weight. Once you register with a number over 26,000lbs the taxes go up in hurry no matter the state.
It's possible in your state the trailer is registered separately from the truck. If this is the case then you need to register the truck at it's full GVWR and register the trailer with the remaining weight. It wouldn't hurt to add some extra since your trailer can handle it. Just make sure you buy enough registration for your combined weight and you should be fine. The newer F350 SRW truck are rated at 11,000(iirc), and would tow your 14,000lb trailer fine if don't want a dually. Another possbility is having your trailer de-rated with a new GVWR at 15,000lbs, which would put you under the 26K CDL threshold combined with a truck rated at 11,000lbs. It might save you a lot of hassle. I had 1 axle removed from my tri-axle trailer(derated from 18K to 12K)in order to get below 26K. The trailer place had no problem doing this since they were the manufacturer.
Here in GA you buy we buy registration based upon the weight of truck AND trailer. The basic plate or registration starts at 14k, then goes to 18k, then 26K, and up from there. All license plates 18K and up have the weight written on the plate. This makes it very easy to spot a truck which isn't registered correctly.
Hope this helps some.