I love this idea, RIFTS is one of my favorite settings.
For the Shard plate first decide what MegaDamage you want a Shardblade to be capable of. Then compare that to how many hits shardplate takes from a shardblade. It seems to me that a single plate can withstand 3-4 direct hard hits. I would do the limbs at that amount and the chest at double or triple, maybe the helm at 150%.
Example:
Shardblade : 10 MegaDamage
Shardplate:
Arms 30 MD
Legs 30 MD
Helm 80 MD
Main Body 120 MD
This is my 30 second thought, you might want to think about it a little more. You also need to consider the effects of a shardblade in addition to its base damage.
It would be awesome if you posted your work here as you create your classes.
