you need a spindle length that will clear the chainwheel (the front sprocket) from hitting the frame and give you a a straight chainline (cross check a singlespeed??) and no, you most likely won't feel the difference of 2.5mm, if it's a fixed gear and you ride aggressively you might want to find some nice 165mm cranks but you wouldn't need to replace your bb, be careful messin with your drivetrain if you dont work on bikes that often..