Hope these questions aren't too stupid

I am wondering whether or not I need to get my pistons decked as I have had the head skimmed(can measure thickness if req'd) and I am a bit unsure of how this will affect the compression ratio, I have a st2 conversion which was fine till head gasket went pop (found minute fracture in block so brand new 200 block was bought).

I am also unsure as to which head gasket to get, I have heard bad reports from tuners re-cometics and some advise gp A and some say to use Multi layer to sort compression.
Also told to use oe stretch bolts instead of arp's!

Any advice or help will be greatly appreciated

Steve