Pro taper means the cue is straight or tapered very little , for the first however many inches. Typically a pro taper goes anywhere from 8" to 15". The shorter the taper the stiffer the shaft typically. Break shafts usually have the shorter pro tapers. Ive always used "straight" taper synonymously with conical taper. The taper starts at the tip (whatever size it might be, 11.75mm, 13mm etc) and goes to the joint. Think Z shafts, carom cues etc.
All of that said, there is no standard measurements for any of it. Different makers use different tapers etc to make their cues have a signature hit. Schon for instance uses a 10-12" taper, where Meucci's is closer to 15".
For break cues, I dont know if a conical taper would best, as Ive never seen one. Usually youll find a short, stiff pro taper on break cues.