I hope you get some feedback soon from a more experienced grower. I only have a year's experience with dahlias.

I think those pots are about the right size for the plants you have. Something a bit shorter might be a bit more proportional to the size of the plants you have, though, since I don't believe that the ones you have will grow all that much more in terms of height. I *think* those are bedding-type dahlias that are bred to stay small. The fact that they are flowering already at that size also makes me think that they are at about their maximum height.

I wouldn't add any other plants as companions in those pots, but that's just me. I have a hard enough time keeping my dahlias watered adequately when they're planted in the ground, so I would think they'd want all possible pot soil volume to themselves, for water and nutrients.

Good luck! They are really pretty.