According to the Wikia page on Mass Effect 3 power combos, ammo powers (incendiary, disruptor, and cryo) can prime combos as rank 1 powers in multiplayer mode (emphasis mine):
Note that for purposes of combo levels, multiplayer Ammo Bonuses always count as rank 1 powers. This is in contrast to single-player ammo powers, which use their respective power rank for the sake of calculating combo level.
A consequence of this is that ammo powers are sub-optimal combo primers, since at most you'll only be able to do half as much combo damage as you would be able to do by using a rank 6 primer and a rank 6 detonator instead.
My questions are,
- Can ammo powers override any existing primers that already exist on a target?
- Can ammo powers prevent other powers from priming a target, if the target has already been primed by the ammo power?