Very wide field to aswer on all questions in one post. Try to read something there are a lot of books on EDABoard.
All ESD protection elements can be divided by 2 category:
1) Breakdown: e.g. TFO (thick field oxide), GGNMOS (gate grounded NMOS), GCNMOS (gate coupled NMOS), LV/MV/HV SCR (low/mwdium/high voltage silicon controlled rectifier) etc.
2) Non-breakdown: e.g. diodes in forward direction, RC-timed clamps etc.
All of them are differentiate by breakdown voltage, snapback voltage (Vt1) etc.
The first of all don't use diode in reverse direction as protection element. It always has low current capability.
NPN device (snapback action) is often used. It has moderate current capability per area unit. TFO, GGNMOS and GCNMOS also can be called as NPN device.
SCR device (also snapback action) is very sensitive to process, can be triggered by rapid change of voltage, usually HV. But it has the best current capabily per area unit. If u don't sure in this device it's better don't use it.