torx or star

Two questions I need answers for please: 1: I am trying to remove the flange bolts that hold the drive shaft to the diff. A T50 Torx bit is a poor fit and has slipped. A T55 torx bit is way too big. What should I be using? 2: I am struggling to find a replacement. So far my options are...

I am going to attempt to change my glow plugs on a '99 C250TD and i understand that the inlet manifold has to be removed to get at them. Can anyone tell me the size of torx key i need for this please? Also I have heard scare stories of the plugs snapping regularly when trying to get them out...