The "military" mics are as stated dynamic, balanced (each wire is balanced either side of ground to cancel unwanted RF pickup) and most importantly cannot support the Electret or Carbon style mic because they have no DC feed to the mic, effectively shorting it. Their audio output is low, typically 3 - 5 millivolts
The twin plug US mic system was originally designed for high output carbon mics producing arround 100 milivolts, with a DC feed to activate the mic. These were replaced initially by dynamic mics which needed an amplifier of about 100x to match the radio, the DC feed drives the amplifier whilst an isolating capacitor is needed to prevent the mic being shorted out. The Electret effectively has an amp built in, though extra amplification is usually required, they are simpler and cheaper that the dynamic type; to produce, not necessarily to buy!
Mixing and matching these devisces generally doesn't work because one invariably shorts out the other stopping it from working altogether.
On the headset side, the impedance is not so critical, most will work anywhere, only the volume level suffers when different headsets are used. The military system uses an impedance of arround 75 ohms whilst the US two plug system is closer to 300 ohms.