there are also subsystem target controls on the bottom of the weapons panel that let you select your target subsystem for your next shot, which are engines, weapons, shields, or auxillery, and if you use them you get a % chance at disabling that system. I usually dont use them much because there's no key bind to them which makes it difficult to use when you're focusing on flying and shooting.
You can place them in one of your trays and assign keybinds using the Keybind tab in Options.
I generally just click on them when I need them though. I have too many other things to worry about in my trays.
Tactical captains of the rank of Commander and up and Tactical BOffs can target subsystems. The Tactical captains get the whole array at once, while the BOffs are limited to selecting one or two.
I don't know about that Science ship comment. I'm a Tactical Captain who flies a Science ship, but I'm pretty sure it's the fact that I am Tactical that I have the subsystem targeting ability and not because of my ship.