Of course they can have different game modes.. I do this all the time in my regular server.
Many people play survival but I, as admin, fly around invincibly in creative mode. There are more players that have been promoted and can play in creative (I'm not the only one), but the regular players are all in survival mode.
EDIT:
I DO think however, that player gamemode should ONLY be stored when it's different from the world's gamemode otherwise when you decide to change a world's gamemode to something else, all old players will stay in the old gamemode and only the new players will go to the new gamemode.
Maybe store gamemode as "default" in such case.
Many people play survival but I, as admin, fly around invincibly in creative mode. There are more players that have been promoted and can play in creative (I'm not the only one), but the regular players are all in survival mode.
EDIT:
I DO think however, that player gamemode should ONLY be stored when it's different from the world's gamemode otherwise when you decide to change a world's gamemode to something else, all old players will stay in the old gamemode and only the new players will go to the new gamemode.
Maybe store gamemode as "default" in such case.