I can't think of any reason to remove the snapdagon. It sounds like a good pairing. All of the roses I grow in containers share the containers with companion plants. Some of the happiest are the roses with a clematis planted in the same container. Other good companion plants are dianthus, verbena, hardy geraniums, petunias, and spring-flowering bulbs, epecially freesias, tulips, and ixias.
Size is important, of course. The container has to be large enough and the companion plants have to be small enough to preclude serious competition. Even when I'm temporaily growing young roses in small containers, though, I'll plant an impatiens or some lobelia in the pot. There's a practical reason for this: Impatiens and lobelia are quicker than a rose to show the effects of underwatering, so they remind me to water those small containers more often than the large ones.
