The SAC is going to be MUCH too big for planting with a Hydrangea. These grow to about 40 feet and become monsters, so I would suggest a long fence instead.
The Niobe will be much more contained in size.

To plant any clematis you will need manure, bone meal and compost to mix into the soil.
Here's what I do:

I dig a 1 1/2 x 1 1/2 foot hole, remove the dirt from the hole into a wheelbarrow and then discard 1/2 of the soil along with all the rocks. With what's left, I mix in a lot of manure, 1-2 cups bone-meal, compost and any good garden soil I can find to replace what I threw away. Mix very well and plant the clematis 2-3 inches deeper than it was in the pot. Fill with the mixed dirt and water very well. Keep watering until the dry season is over. Good luck.
Suzy

Someone here suggested that after digging the hole, make a cone of dirt and cover the dirt with wet newspaper (to give it support), then carefully spread out the roots of the clem and place them over the cone. I tried it a few days ago, great way to get the roots spread out.

Patricia

Climbing hydrangea can reach heights of 50 to 80 feet if it has the support of a large tree to do so. It looks great on tall trees, on stone pillars, on brick walls or cascading over a rock pile. It is not overly aggressive like some vines can be. Dr. Michael Dirr, author of the Manual of Woody Landscape Plants calls it "The Best Vine!"

Climbing Hydrangea is a great plant that has tremendous potential. Few people have ever seen this plant. Once they have they will never forget it and will most certainly want one of their own!

The above is a paste from another website. I know it (the Hydrangea) will not grow as fast as an SAC but should give a great support for it to grow up on eventually. I think?
Anyway,the planting instructions are great but the only question it leaves me with is when can I plant it.
Should it go in the ground now or should I put the whole pot in the soil and wait until next spring???

My knowledge of clematis from the forum & reading for many years is NOT to disturb the rootball. You may loosen the very bottom of the roots if they are pot bound [be gentle] but do not disturb the whole rootball. Clematis do not like their rootballs messed with at all. Also I have learned that planting the rootball 3-5" deeper than in the pot does wonders. All my clematis have been treated this way & all are huge. It may be a area to area thing but spreading a clematis rootball I would never do.

I am sorry, I missed the word 'Climbing' when you said you had a Hydrangea. I was thinking of the bush. As usual, I was in a rush.
Climbing Hydrangea will make a fairly good support for the vines of a clematis, however there will be some major root competition as the cl. Hydranges mature. Cl. Hydrangeas have very large and invasive root systems! I have a tall fence covered with them and a flower bed in front. The roots have to be removed from my flower beds annually. Some are 2-4" in diameter with many smaller roots near the surface serving as 'runners' for new vines. Clematis planted directly in front of them may have a hard time competing in a few years, but still, I too have planted a few group 2 clematis to climb my own cl. Hydrangeas! If they begin to look weak and spindley I'll move them. I look at this combination as an experiment in my own garden.
I'm doubtful about placing a Sweet Autumn with these Hydrangeas because of it's agressiveness. It will probably outgrow and completely smother your Hydrangeas from view in just a few seasons.
Suzy

Suzymac,

You would'nt happen to have any pictures of your cl. Hydrangea and clematis combos???

My clematis are still small, (they were my own seedlings), so I haven't taken any pictures of them planted, however, this picture shows the Climbing Hydrangea in the rear of the picture covering my fence. You can get an idea of the bed in front as well. These Hydrangeas are now MUCH bigger and are climbing the trees behind the fence. The roots are enormous.
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also a picture showing the Climbing Hydrangea in flower in the top left:
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Your a professional are'nt you?? Your Garden is stunning. I wish mine looked like that!

No Jodonne, I'm not a professional at all, just a gardener who has been serious about gardening since I was a child. This garden changes every year. I move things around and add other things yearly. I can assure you that all this didn't happen overnight. This garden has taken me 20 years to reach this stage. Since this picture was taken last year I have planted many new clematis and moved the blue Hydrangea you see. Things tend to overcrowd as they mature, so I try to keep things in check and move them when necessary. It's a constant job.
I wish I had more pictures of the cl. Hydrangea for you, but these are the only pictures I have which even show them.
Suzy

Suzy.. Very pretty...I had read somewhere that the SAC vines themselves tend to "Dig" into and can ruin some structures...is this true?..I don't have a SAC...Hence I would think the SAC Vine itself would kill off the Hydrangea?..What say you...Jeanne

Jeanne, my SAC vine does not dig in to structures (at least not yet!), so that isn't a problem with a climbing hydrangea, the real problem lies with the fact that the SAC will totally cover the Hydrangea and it would smother it in time. But, cl.Hydrangea's DO attach themselves to structures and might cause problems as they mature.
Suzy

I'm thinking I need to abandon the Hydrangea and SAC combo. And that's fine,but what do I do with the SAC now?

Got a fence? A small tree? A pergola or a hill ?
Mine is growing on a large arbor which it has covered.
Suzy