Dear Tobias,
1. Dhamma are rupa too:
What are rūpa? – Dhammā are rūpa too!
2. Also, a living being is ALWAYS associated with a a rupa condensed above the dhamma stage. Even an arupavacara brahma has a hadaya vatthu, which is a suddhashtaka.
So, a given “lifestream” is always associated with a rupa at or above the suddhashtaka stage, in addition to dhamma.