Timo Oh, I did not know that the original plot called for the Romulans to be using industrial espionage. Was Wah Chang ill-compensated for his work? The Romulan Bird-of-Prey was created before the Klingon D-7 design? Especially if you insert the industrial espionage angle... Industrial espionage was though... From what I remember in that episode Scotty said the Romulans only had basic impulse. However, the ship did get pretty far out into space (and traverse a considerable distance even in the episode, if I recall) -- to the point where warp-drive would be the only realistic explanation. That and the fact that it has Warp-Nacelles. When you say "outwarp" do you mean "out-do"? From what I remember the Plasma-Torpedo was a sublight weapon* -- and would appear to be a ball of plasma hurled out from the ship (the plasma-ball was transparent and did not appear to have any solid mass in the center), although it's not even clear in TOS whether the Photon torpedo was supposed to be a solid mass either (In fact I've heard some explanations which said they were matter and anti-matter seperated by magnaphoton-fields, and they simply looked like comets shot out of the ship) and was only made clear in TMP when Chekov stated that they were taking on photon-torpedoes while the ship was in spacedock (However even the special-effects guys still largely assumed they were just balls of plasma, energy and deadliness) and visually seen the first time (just the torpedo by itself) in TWOK. CuttingEdge100 * - While technically a stream of plasma flung out of a ship cannot exceed the speed of light, technically a ball of matter and anti-matter with magnetic-energy fields sperating them couldn't exceed the speed of light either -- however Photon Torpedoes were routinely fired at warp-speed, as were the phasers (which only traveled at the speed of light)
What Scotty says is that the Romulan power generation is "simple impulse" which is not the same thing as their propulsion being simple impulse. All of the available evidence suggests that building a sublight ship as your new weapon would be incredibly silly. The Enterprise had to go to warp to outrun just the plasma weapon's maximum range, so it doesn't seem likely that the BOP wouldn't have had either warp or some other FTL system. It simply wouldn't be practical.
So, I assume by Star Trek terminology they're talking about fusion power? Well, technically it takes time to transition over from impulse to warp... a few seconds... it is possible that the plasma torpedo would be able to hit you before you'd make warp.
I'm not really sure about fusion power. I know in Star Wars fusion engines have appeared in several species, but are described as being less efficient than the ion engines found more commonly. I can't recall if fusion propulsion has been mentioned much in Trek, unfortunately.
Internal combustion is the basis for automobile propulsion, so I guess it can't be used anywhere else. Fusion can and likely is used for warp propulsion. I don't know where Trek gets this silly notion that everyone uses the same power system for their warp drives and the same drive type as well. It's ludicrous in many ways.
I believe the Phoenix, albeit an experimental ship, did -not- use matter/antimatter for warp. You could speculate it used fusion, possibly from derived from the missile warhead. We certainly have had in-universe ion drives and Romulan singularity drives that could travel at warp. There's no requirement for anti matter, just a LOT of power to warp.