afaik, the HP and SP bonuses from Greensteel items stacks with anything else.
For the Insight bonus though it runs with only the largest if you have two. For example: +1 Insight AC(idk, ring lets say) and GS item with +4 Insight AC would come out to +4 Insight bonus to AC.
I am only just getting started on my GS work and have one Khopesh Blank so far only and a handful of lesser ingredients.
The general consensus(though still debated) is that melee's should focus on a suitable weapon first and then armor/clothing and the opposite for casters. Also keep in mind that only one greensteel non-WEAPON item can be worn at a time until they are "cleansed" which drops only on your 20th Shroud run afaik.
Hope this is helpful and sorry there were no sooner replies. I just got my account on the forum today.