you still have the choice in the lobby between medic or ambulance mobile spawn...
Could you make it the boss man?
I'm not sure I'm undestanding...
If you want to play as leader and also manage the mobile spawn, choose "ambulance".
if you're the leader and want to use the mobile man, choose "Alpha medic" , one team mate has to choose that slot...
I don't think it's logical if the medic is the boss: remember that this mission uses "high command" functions, so you can have help from the 3 other squads from the camp... 4 squads leaded by a medic?