If you carry a load out be sure all the other participants have the weps or they will be faced with a list of "cant find the ..." messages as they enter the flight. Having said that, even if you all have the weapons you will only see your own ordinance being fired.
Assuming all your players have the same mission loaded as the multiplayer mission they will all see any AI planes and ground units placed there in, but the units are unique to each player thus, if a player attacks and destroys a unit only he will see it destroyed all other players will still see it undamaged. so if you want to run a team co op mission against an AI target you will have to use Brunhost to play the target.
Moving AI units like ships are likely to diverge in each players game.
keep us informed of how you get on, there are a number of us who are experimenting with what can be done with multiplayer.