I have various perennials alongside mine ..... penstemons, aquilegias, hemerocallis .... all types of flowering perennials actually, but you could have whatever took your fancy - no hard and fast rules !
I have my phlox as a front-of-the-border plant and the taller plants 'behind' it.

Just thought, i also have trailing ones along a rockery where they're with saxifragas and silenes !