I dont have sufficient trials yet but need a fast certification/registration, can I use another PFC and transition to PFC 6 later?

Yes absolutely. Perhaps your product has nutrients or also has a soil improving effect. In that case you can register it under PFC 1 or PFC 3. EFCI Register can help you in this regard, and there is no hinderance to later also, with trial data, adding the PFC 6 Plant Biostimulant certification. 

You can then even sell the product, under a functional blend, as both a PFC 3 and PFC 6 for example (provided you did both conformity assessments.