Sir Zup,
Though I like all formats, including mixing of WWI and WWII planes, but there is definitely speed advantage in WWII Plane as compare to WWI planes. With this extra speed in WWII planes, the flyer has advantage in all types of game, may it be dogfight, BTH or CTF mission. It's unfair for WWI plane flyers without giving them fair chance to compete. It's taking away new joiners and even the old one quits the game in-between as their flying skills are no match against the speed of WWII planes.
SOLUTION:
Let there be 2 Servers WWI & WWII.
If someone hit WWII and there are players in it, the game will go on as it is.
Now the other option, which is more common, the player finds empty game in WWII and hit at WWI server:
As soon as WWII player hit the WWI server, decrease the WWII plane speed to match the max speed of Siemens/Sopwith Camel Fighters. This way the WWII player can join the WWI server without speed advantage. The WWII plays can still hold 4 bombs, if they have.