gryffin,
If you were thinking about color combinations, keep in mind that they may not be flowering synchronously unless you can find enough information on which are considered quick versus slow bloomers.
Personally, I would mostly avoid mixing short (such as Alfresco, Jewel, Amputo) and tall varieties (such as Ruby Star, Evergreen) in the same pot.

Bill

I was originally thinking colors, but it makes sense to me that they may not all bloom at the same time and that heights are what I need to be most concerned about.

I'm having trouble finding sources for the height of some of the cultivars, for example, Evergreen. Many sites just list a range, I found one site that listed all of their varieties as 12-24 in. I guess they they figure that can't go wrong with that wide a range!