You DO NOT need a CDL for an air brake equipped vehicle if:
On a US licence, the supposed "air brake endorsement" does not exist. For a driver obtaining a CDL, they must take the written air brakes test, and conduct their road test in a vehicle equipped with air brakes, or else they will get a restriction prohibiting them from operating a commercial vehicle which is so equipped. For everyone else, no such criteria exists - RVers, people renting U-Haul/Ryder trucks, etc. do not need to know the first thing about air brakes in order to be allowed to operate a vehicle equipped with them, so long as that vehicle doesn't require a CDL.
You need a Class C CDL--you can't put that kind of endorsement on a regular drivers license. These days there's no such thing as an air brake endorsement; there is a "no air brakes" limitation if you come to your road test in a truck that doesn't have them.
26,000 lbs. is what the manufacturer rates it at. So long as you do not let that vehicle get over a gross weight of 26,000 lbs, you do not need a CDL to operate it. The only times you are required to have a CDL for a vehicle under 26,000 lbs. is when it's carrying a sufficient quantity of hazardous materials that placards must be displayed, or if it's designed to carry more than 15 persons (including the driver). If you're attaching a trailer to this 26,000 lb. truck, then you are creating a combination which is rated at over 26,000 lbs. GVWR, and will need a CDL with air brake allowance - Class B is the trailer or vehicle in tow is rated under 10,000 lbs, Class A if the trailer or vehicle in tow is rated over 10,000 lbs.
A CDL is not required until the passenger capacity reaches 16 persons or more (this includes the driver).
A CDL is not required until the capacity of the bus is greater than 15 people (including the driver).
The answers for every state's CDL tests are found in the CDL handbook. If you're not willing to study it and learn these things, well, it's probably for the better that you don't get a CDL.
Not in Pennsylvania, nor any other state.
Yes. However, your chances of finding employment as a CDL driver with a felony conviction are slim, and you won't be eligible to receive a Hazmat endorsement.
Whatever the employer pays their drivers.
So long as the total Gross Combination Weight Rating of the vehicle is 26,000 lbs. or less, and the weight actually remains under 26,000 lbs., then no, you don't. You also don't need a CDL if it's a vehicle exempt from FMCSA regulation, such as a recreational vehicle.