Din, in your hot zone, I would suggest the viticellas and integrifolias for your first adventure into growing clematis. They hold up the best in my zone 7a garden with heat and humidity, although your conditions may be a bit more hot and humid depending on where you live in Texas. I also have mostly clay soil that is only slightly amended when I plant the clematis. Here if you overamend your soil too much (unless you are amending an entire bed), you end up with a planting hole that becomes a swimming pool when and if it rains--this is not good for clematis. The type IIs in my zone get quite brown and crispy once the temperatures get much above 85 degrees and are also more prone to clematis wilt so I would suggest staying away from them.

There are quite a few clematis that will live quite nicely in a pot provided that the pot is large enough. In your zone I would suggest as big a pot as you can possibly get with a minimum height and depth of two feet. Anything much smaller than that and the soil will dry out way too fast and your clematis won't like that. I would also suggest situating the pot so that it gets morning sun and some shade during the hottest part of the day to help in preventing the soil from drying out and the root mass getting too hot. The integrifolias as a group generally stay under three feet and would do well in a pot. You will need to contain them since only Roguchi in that group has any clinging and climbing tendencies. A tomato cage would work nicely as a means of containing them. Konigskind and Piilu are type IIs that would also work well in a pot and have never gotten clematis wilt for me. I will leave other suggestions for clematis for pots open for others to comment on. I have a plethora of books that have lists of clematis suitable for pots. Some of the ones listed I have no experience with and are type IIs which I personally tend to shy away from. Others are ones that I have but am growing in the ground and they get rather large so I question their suitability for pot culture even though I am aware of the fact that things grown in pots can be stunted in height if the pot is smaller than required.

As to whether to purchase large or small plants, that depends. If money and availability were no problem, I would always purchase the largest sized plant I could. Since I don't have unlimited funds and often times have my heart set on a particular cultivar and it is available only in a 4" sized pot, I purchase whatever size I can find. That being said, if the clematis is in a pot that is smaller than quart sized, I always pot them up in one gallon sized pots and grow them out until the roots fill the container prior to planting them in the ground.

Although I wasn't the one asking the question, that was a very helpful answer. Very thorough.
Thanks