Single use motors do not have thrust rings on them like the reload casings do. If you plan on flying HPR (H and above), you'll want to invest in reload casings. I would say the same for midpower, to tell you the truth. Cost per flight is much lower with reloads.
As to the rest of your question...Since single use does not have a thrust ring, you'll either need to make one on the motor with tape, or install some sort of thrust ring. The method described with installing one for the longest motor you'll use then use adaptors is a good one. If you plan on using reloads, you'll only need to have some sort of retention method to keep the motor from kicking out of the rocket when it firest the ejection charge.
As for motor lengths, larger letters typically are longer motors. But not always. I believe the Aerotech motor matrix has the motor lengths on it...