Here's my bit...
There ought to be two different kinds of crates for each set.
(e.g. Large Ammo Crate, Small Ammo Crate, ...etc)
The small crate would be enough to supply a soldier's portion.
The large crate would be able to supply a handful of soldiers.
Now, for spawning. Perhaps, the server controller can decide if it is appropriate to have auto-spawned crates or player-spawned crates. So, along with team colors, respawn time, and game type; it could be determined whether crates will be spawned automatically or spawned by players.
For the Auto-spawn crates option, there should be a predetermined time when a crate would be released. Longer times for Large Crates to appear. Perhaps a random timer with a certain minimum time limit?
For the Player-spawn crates option, there should be a cool-down/recharge time, but that time can be shortened with consecutive kills (e.g. 3 kills -3sec, 5 kills -9sec, etc...?) . Large crates would take longer to recharge.
I also like how some people have the idea to use the type of weapon the player uses to determine the drop type.