How thick should my concrete walls be?
ICF block comes in core sizes of 4, 6 and 8 inches, and the insulation is 2.5 inches per side. Most of the ICF houses I've worked on were exterior and load bearing only. In Ontario, other jurisdictions may be different; any time you're dealing with load bearing walls you need to have an engineer certify it and submit it to your local building authority. I've never heard of any engineer putting their stamp on someone else's drawing (I'm sure it does occasionally happen) because they are absorbing the liability for someone else's work. 

From my experience, foundation walls are always 8" + 5" for the insulation and I've seen 6" on upper levels. The 4" blocks, I would imagine, are for partition walls. They are very comfortable houses to live in. The rated insulation is pretty much what you get, as opposed to a wooden structure. With lots of extra framing and pack points, along with poor insulators and difficult spots to insulate, the actual R-value of your typical wooden structure is far less than the R-value listed on the bags of insulation or what the spray foam guy is telling you. Wood is a poor insulator.

ICF is also very tight so a HRV or ERV in a warmer climate is a necessity. Also, consider concrete floors over corrugated metal with radiant floor heating, if you are in the north. The drywall can go on a stud wall directly on the face of the block. Most seem to use steel studs. Hope this answers at least some of your questions.