@MotoBlue Yes, you can configure the nameserver that is used in the dhcp-server. 

I think the guest network devices dont have access to devices on the main network, so I don't think they can get to your pihole, so they should get the main nameserver configured at your internet settings (so either an isp one, or a manually given one).

  @jijookhier  Thanks for that reply. Do you know if I can define the IPv6 DNS server?

for the isp side you can.

For the dhcp server I don't know, but it might work. But since ipv4 nameserver will respond with ipv6 addresses, I never tried and wanted maximum compatibility for all devices.

screenie