I hope you meant keeping the liftgate ajar while the vehicle is parked, not in motion. As Wilyumm (and your owner's manual) points out, driving with the gate open allows exhaust gases into the passenger compartment.
A slightly opened liftgate probably won't do much for air circulation and would certainly compromise security. You'll get more benefit from leaving the side windows open (admittedly they don't open much). Even cracking the front windows will help with heat dissipation, but if you plan to leave the dogs unattended, even for short time, consider putting Reflextix in the windows to keep the sun out.
If you filled in your profile, we'd know whether you have a skylight. Removing the skylight completely will do the most to allow air to circulate and heat to escape.
