If you start with ST, that gives you an idea of available weapon size and power. With that information you can move back to armour to get the effects you want.
So for example, if a Scout class heaviest weapon does 6d6 (5) and you want Tank class to shrug off Scouts like they're mosquitos, Tanks need 105 DR to have a 50/50 chance of taking no damage. So that implies that Tanks probably want a minimum of 150 DR on any location a Scout can either reasonably target or hit by accident.
Various people on this forum have done a
lot of work on weapons and armour, you should be able to save yourself some time by trawling through searches.
EDIT
Also there's stuff on mecha specifically, for example this:
http://forums.sjgames.com/showthread...ighlight=mechs