The X200A and X200B model are similar in design, are both LED's, and will both mount to a picatinny rail. The difference between the two is in their output. The X200A provides a 3 watt tightly focused beam whereas the X200B provides a 5 watt beam with much wider spill, meaning that instead of a tightly focused beam there will be a much wider angle of light output. The X200B is brighter but the X200A will allow further engagement/identification. Generally it's a matter of personal preference. Depending on your environment an X200 may be an ideal match for your rifle, but I generally suggest an M600 Scout Light, or any of the 6 volt millenium lights for pistol caliber carbines.
As for the P101 and M101 those are designed as handgun mounted lights and I can't say for certain that attachment to a pic rail is possible. I arrived at Surefire after the debut of the X200 and have dealt only occasionally with the other handgun systems. And Nitrolon refers to the material the P101 series is manufactured from. The bulbs used in both lights are standard 6 and 9 volt Surefire lamp assemblies. The M system is a 9volt, meaning 3 SF123 batteries, whereas the P is a 6 volt so two SF123's. If it is still your wish to mount an M or P101 to a Pic rail I will check if you'd like.
Regards,
Alan Elam