...nothing like real space combat.
Two things: one, we don't have such a thing yet, so it's hard to say that.
That's true, and some technologies would really change things drastically, like instantaneous warping through space instead of linear travel. You'd have ships jumping around each other trying to get a shot. However, if we go too far in that direction you end up with basically magic tricks, and anything is possible (that old Arthur C. Clarke line about high technology). If we don't project too far ahead, we can explore ideas about faster speeds and longer distances that might at least be possible one day.
The one thing I think is a safe assumption, is that as speeds increase, distance between opponents will increase along with it. Think about the change in dogfighting tactics between WWI biplane gun duels within spitting distance, and over-the-horizon targeting with modern jet fighters. The tactics changed because the speeds, weapons, and sensor tech changed, increasing the distance of engagements. I don't see why that wouldn't happen to an even greater degree in space. It just makes no sense that ships would be capable of interplanetary or interstellar travel speeds, but they'd be less than a light second or two distance from each other (which is REALLY CLOSE in space) during combat.
Two (and I suppose more, as this is getting long now that I'm typing it), When it comes to space weaponry, the size of missiles and mass drivers would be constrictive. Beam weapons could (with theoretical reactors) theoretically produce a larger bang for much less mass, allowing the ship to keep more for movement.
That's true, although conserving mass wouldn't matter if it means you win, and the other guy is a plasma cloud.
Plus, beams travel at lightspeed and missiles don't.
To target a beam, you need to know where the target is. You can't know that, when the distance gets beyond a few light minutes. A missile can be fired semi-blind in the general direction of the target, and home in if it's faster than the target. Taking advantage of a beam weapon's speed means you have to be very close (in terms of open space)... just a few light seconds away. The enemy might be able to do a lot of things before you get that close, like sending a seeker missile your way, or just flushing all the toilets to create a cloud of debris in your path. At those speeds, you really don't want to hit anything.
Plus, shooting a beam doesn't result in (nearly as much) reaction force displacement of the ship as shooting a missile or mass driver. I'd say it's similarly possible that "real" ship combat may well turn out to be a close-range affair, primarily with beam weapons, resorting to old fashioned naval broadsiding tactics. Eventually reactors and engines would become efficient enough that maneuvering engines could come into play on a close scale. These engines wouldn't be for going fast, they'd be for allowing a ship to shoot an enemy without allowing the enemy to shoot back. Think, similar engines to the rockets we use on the space shuttle now, just likely in a more efficient format. Cruise engines would obviously be for travel only.
Well, I see it more like modern submarine warfare instead of a close-range cannon duel. Why give up the advantage of stealth? Distance equals stealth in space, or at least delayed response, due to the lightspeed barrier.
On the subject of nuke damage... if you can get the nuke close enough to the target, a better use for it might be to pump a one-shot X-ray laser and cause some serious localized damage. That would be a hybrid between the beam-weapon-on-a-ship approach, and a missile warhead, with the advantage that a fast missile can get past the lightspeed "fog of war" and find the target.
