Guernsey Cream is a type 2 prune, so would be easier on its own since it is difficult to separate the vines for pruning if you have two different pruning types growing together. Mine is probably 8' or 9' tall after 5 years, but it is in deep shade for most of the day and still manages to bloom well. I am not sure how a GC would do with better growing conditions. Some type twos you can prune like a type 3, but I am not sure if GC is one of them.

Betty Corning a beautiful, but quite large. I don't know sizes on any of the others, but I would look them up to see relative size and pair ones with similar size and vigor. If there aren't any of similar size to Betty, I'd put her on her own trellis as well.

I have Etoile Violette and it a vigorous clematis and would probably hold its own with Betty Corning.

thank you both for your feedback!