Any abrasive can cause damage to the nail plate depending on how it is used. a 3 way buffer will cause damage if misused.
The numbers used in conjunction with files and buffers refer to the size of grit. A low number means large particles that will deeply scratch a nail plate with just 1 swipe. A high number is smaller particles that won't deeply scratch the nail plate unless more pressure/strokes are used.
240 is the lowest number that should be used to prepare the nail for a good quality overlay as very few gentle swipes is all that is needed and causes least amount of damage. It is only the surface layer that need preparing no deeper!
White blocks have different grits, some harsher than others. Feel a 240 grit file and feel a white block, if it feel the same that block is absolutely fine to use. Quite good actually as they are so soft with the foam. You just need to make sure you get down the side walls but this is easily done with the corners of the block