You dont need a CDL unless the vehicle or combination is over 26000lbs. PERIOD. Doesnt matter what the trailer GVWR is, as long as the trailer and/or truck arent over 26000lbs.
http://www.dot10.state.pa.us/pdotforms/pub_223/section_1.pdf
Straight from PennDot:
Class "A"- A combination of vehicles with a GVWR of 26001 lbs or more, provided the vehicle being towed is in excess of 10000 lbs.
Class "B"- A single vehicle with a GVWR of 26001 or more lbs.
The key is 26001lbs. The only reason the 10000lbs trailer is mentioned under the class"A" is to distinguish between class "A" and "B".
You are right that its federally regulated though.