The front seat frames are different '65-'67 vs. '68.  Starting in '68 Ford used a locking mechanism for the seat back, which required the frame be changed to accommodate it.  This was a crash safety feature like the collapsible column and side markers which also started in '68. 

The rear seat frames are the same '65-'70.

The upholstery changed year to year, and with certain options (pony vs. standard, deluxe vs. standard).

The rear seat upholstery will definitely work.  The front upholstery MAY work.  I can't say exactly how different the frames are, and whether or not this actually effects the upholstery beyond some holes you'd have to cut in it for latches, etc.  That said, the style is not the same '66 vs. '68, so you would have to decide if that matters to you.  Personally I special ordered my TMI front seats to get the correct for '67 horizontal ribbing vs. the vertical ribbing every aftermarket seat seemed to have.  That way the fronts match the rear and the door panels.

Can anyone confirm if 66 standard skins will fit 68 standard seats?

I found a pair of '70 Mach 1 high back seats in a junk yard many years ago. The side windows had been removed (probably some guy with '69 glue-in glass swapping to '70 bolt-in glass) and the carpet and seat foam was soaking wet. The seat bottom frames were completely rusted to junk. I soon discovered that my '65 seat bottoms were exactly the same as the '70 seat bottoms except for the latching mechanism. I bought foam and covers for the '70 M1 seats. They fit perfectly on the '65 bottoms. I simply removed the latching mechanism from the seat backs and the seat covers were not cut for the latch handle shaft to stick through. So now I have '70 M1 seat backs installed on '65 seat bottoms and other than the missing latch nobody can tell the difference.

Not sure, I would call TMI.  That's what I did when ordering my seats.  They are a good company.  My gut says even if they don't have a "stock" solution like what you want they can probably make it for you.  A lot of their stuff is made to order anyway.  Typically their custom upholstery is designed around their foam, which is designed to allow you to use the stock seat frames while adding side bolsters, etc.  Basically its cheaper than a full seat or custom upholstery.  Personally, I'd really think about how you use the car.  For years I drove my '02 WRX which has seats with good bolstering and support.  Going back to my '67 I realized how much I was sliding around in the seat when I pushed the car.  The new TMI seats are much more comfortable than stock and provide much better support when hammering through twisty roads.  I couldn't imagine taking a long car trip with the stockers.  The lack of support just didn't jive with the way my spine is configured.