High DEX and Armor

[PseudoPanda]

BTW, can I use both an armor and a Ghost Shroud?

No, it takes up the body slot which is the same slot armor takes

1. Does being immune to cold damage makes my weapons and armor immune to it as well?

2. The armor has a natural 5 AC when it is targeted directly, plus my DEX and deflection AC. At level 20, this would mean 5+18 plus any number of deflection bonus I manage to get. So, is 23 AC plus a hardness of 22 (10 base+2 dwarvencraft+10 enhancement) plus 150 HP enough at level 20?

1. No, the armor stops working based off of whether or not you fail your save not how much damage you take.
2. AC 23 is pretty low at level 20 and hardness is not too hard to bypass

Can I change the materials that make the armor?

Since it's a specific armor, probably not but I'm not sure if there's an actual rule to that so ask your DM