In the Zaino system Z6 is completely compatible with any other Zaino product. The same is true of Z8.
That said you are correct not all QDâ€â„¢s can be used prior to Z2. It all depends on how you define what a QD is. If the QD in question contains any type of oil, silicone, carnauba, etc., then it will interfere with the Z2 bonding and your durability will suffer. Much like you are advised not to use any type of polish that contains oils, fillers, carnauba, etc. without first doing an alcohol wipe down.
Z6 to your content and Z2 anytime you want.