Leave the empty group defined as the VPN domain on your gateway object and the objects representing your route-based peers.
However on the Participating Gateways screen of the VPN Community object for your domain-based VPN, override the VPN Domain definition for your gateway and the object representing your domain-based peer. Try to make these defined VPN Domain overrides as specific as you can; they should exactly match whatever rules you have permitting the traffic to/from that tunnel. This will minimize the chance of disrupting your existing route-based VPNs.
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com