Tim:
Well, first of all you are absolutely correct not to embed the posts into the concrete. Not only should untreated wood not be embedded, it simply should not come in direct contact at all with concrete as it will rot. As pointed out prior in this thread, a yoke or some other metal fastener is most appropriate in this situation.
Your second question should be answered when you have your frame engineered. It is not possible to determine whether it is overkill without establishing the loading of the timbers. Specifically the wind and snow loads, the pitch of the roof, and the spacing of the bents and purlins. In addition you will need to provide the species of the timbers. These questions will resolve timber sizes for bending, any point loads can further impact your size choices. Once you have the size established then you need to determine your joinery. It is not uncommon for the joinery to need a timber that is larger than that required to carry the load.
That said, if we assume Oak (midwest) and 40-50 psf snow load (CABO), 10 psf dead load (shingles on panels), 48" purlin spacing (panels), and a 15'6" bent spacing then your purlins would need to be 8x12 and you would still have close to a 1/2" deflection. So failure is the issue, not overkill! However, if you reduce the bent spacing to 11'6 then a fully dimensioned oak 8x8 would just barely work, nominal fails, that's how close it is. As long as the purlins pass over the principal rafters with minimal notching you might pull it off. Watch out for horizontal shear, shear kills more designs than bending. Unfortunately the 8x10 principals appear to be woefully underdesigned before beginning to address the joinery.
Don't forget bracing, wind loads, and other factors such as roof materials, and "low roof" conditions. There are also some signficant joint loads at the bottoms of the principal rafters, and possibly at the top and bottom of the king posts. This is only an opinion and is by no means an accurate engineering analysis. But as I said at the opening, your engineer will answer all of these questions for you.
These forums are wonderful for discussing these questions but they can not be used as a means of saving money on engineering fees. In the long run the costs resulting from a poorly designed frame greatly exceed the modest fee of an experienced timber frame engineer.
