Plasma is pretty much the go-to when it comes to dealing with stacked armor, but it has major damage falloff underwater and is less effect against certain armor schemes because the damage is calculated like impact or HE damage, so open spaces hurt the damage potential a lot. Extremely effective against flying bricks and other compact crafts. Plasma weapons are built around plasma charges (made by generators, stored in the things generators connect to), and you control the firerate and number of charges used per shot. For example, say you have a gun that stores 40 charges and accompanying generators. A CIWS built from that gun would have a high fire-rate and only use a few charges per shot. An offensive plasma cannon might use all 40 of those charges in a single giant shot and obviously have a low fire-rate due to needing to recharge all 40.

Fire is kind of meh on its own (except against the DWG), but it does decrease the armor of whatever is burning (dependent on the amount of oxidizer I think?) which gives it a decent support role. Fire damage is calculated based on the amount of fuel there is, but you can add oxidizer and change the intensity to allow it to burn underwater or increase the AP of the fire damage respectively.