Foundation Thickness

I have so many questions. My next one deals with the thickness of a foundation. I'm planning to use 4.5 inch SIPs and my posts (which will rest on the deck) will be 8x8's. Does my foundation have to be 12" thick or can I go with 8" thick walls and have the SIPS hang over the outside edge of the wall (and seal underneath them) or should the posts be only partly supported by the foundation? (even though there will be solid blocking between them, it seems the later would put too much stress on the joists.)

I often cast a bearing pier (footing too) on the inside of the stem wall at each post location. A bit more hassle forming up, but makes a cleaner detail, both post & panel have full bearing.

You haven't entered into your profile where you are.
It's hard to say, what the code for your area, would say.
You should have more support at the foundation for your posts, such as you have mentioned.
You could contact your sip provider and get their recommandations for sip support. Most like some direct bearing on the foundation.
You might be able to get away with making a 10" thick wall for 2" of support for your sip if the sip isn't going to carry any vertical load. If it is going to carry some load then both sides of the sip have to sit on the shoe at the foundation.
See if the sip company has a foundation detail drawing, most do.

When I built my timber frame cabin a few years back, I built an 8" thick foundation wall and installed a 2x12 sill plate cantilevering 4" beyond the foundation to support the SIPS. The 8x8 posts bear directly on the foundation. My intention is eventually to face the foundation with stone which will reduce the cantilever but I haven't done that yet and the detail looks fine and seems to be working OK.