Agastache, Salvia - its all in the family. If you look back you'll discover many discussions of other plants. We may be collecting salvia, but we're also growing gardens full of companions for our collections.
Your Agastache mention sent me out into the back garden just ahead of the snow storm this evening and got me looking at the shadow of Agastache x Firebird. Do you grow it? A friend gave me one last year and it was planted at the foot of Salvia microphylla hybrids. It was a impressive plant, growing into a low wide bush heavy with small leaves, full of flowers from mid summer, and even naturally layering itself without help from the gardener. Some catalogues list it as Agastache cana, but it looks nothing like that to me. Maybe there is some cana in its past; and it seems like there could be some rupestris too, because its leave are small and grow fairly tightly spaced on the stem. I could be off base about that, but it gives a picture of the plant. I'm kicking myself that cuttings weren't taken last year, I'd like to use it at the feet of a row of Salvia greggii Wild Thing. Firebird has one of those pink lavender flowers that should echo the lighter Wild Thing and is small enough not to overwhelm it.