About the cribbing sizes, it really depends on the timber framer. If he has a regular supplier of timbers then this supplier has probably made him many different sized timbers before. It is not that unusual to have 5x9 or 4x7's or other none traditional sizes in a timber frame stock list.
And to make a sale, these days, I'm sure a mill would make some 6x8's and cut off an inch of 1x6 and keep for themselves to make up an order of 6x7's.

I have made some cribbing before for jacking up a camp and sold this cribbing twice. Once to the customer; and after he was done with this I helped him sell it to another customer who does timber framing restoration work. It was white pine.

Some companies like hemlock for cribbing as it is very rot resistant as well.

I have made cribbing out of oak for highway bridge repair companies, many times. That stuff is heavy....