I would highly, highly recommend going “Three-Turns per Round”.
Axis Powers go first.
Then, Russia & China go.
Finally, Allies go.
If you have 8 players that know what they’re doing, this makes a very intense G40 game happen in about 7 or 8 hours.
Axis and Allied forces can attack together. Russia may not attack with Allied forces. China also may not attack together with Allied forces.