ram scoops work by putting out a huge 'scoop' either physical or magnetic which collects all the tiny amounts of matter in space and uses them as reaction mass. The act of catching them is what creates the drag. The question is purely whether one can throw them out the back faster than the speed you caught them. If you can then your aggregate force is positive and you have acceleration. Note that you have to get up to a pretty high speed first before the ram scoop becomes practical. And once up to high speeds it does not take a lot of matter in space to create drag. And believe me if you have double digit g's acceleration on an interstellar voyage you are going to get up to seriously high speeds (relativistic even) and you are going to have some serious drag.
I'm afraid I still have to disagree. If you are defending a strategic site of some sort (planet, space station, wormhole etc.) then you will almost certainly have some fixed defences and those weapons will typically be bigger than what can be carried practically on a spaceship. If you are attacking such an installation the one thing you absolutely would not do is slow down to give those defences an easy shot at you. Your only real advantage over the fixed defences is your manoeuvrability and you aren't going to make it easy for them.
On the other hand your targeting is going to be relatively easy, fully controlled by computer, and that computer knowing about each duck and dive you might do, it could quite easily target the fixed installations whereas they, not knowing before hand how you are going to duck and dive would be at a disadvantage. And the mobile defences would want to be on the move for the same reasons. I guess the main question is how fast and if someone is shooting at me I sure as heck would keep moving just as fast as I can, leaving my automated weaponry to compensate.
Actually if we are talking in system battles I would imagine you are more likely to lob missiles at each other from behind some cover like a planet. I've never understood why so many space battles seem to need to 'aim' their missiles at a visible enemy; smart, stealthy missiles would do all that for you.
Whilst on the topic of weapons I would personally expect laser (or laser type) weapons to be more effective than missiles for space battles. By the time a missiles reaches you there's a good chance you might have destroyed it, confused it or dodged it. With laser you get no time to do any of those things. Also the inverse square law hardly applies to these devices. That law refers to a point source where all the energy is dissipating over an ever increasing sphere. Lasers are very tight beams of energy (near as dammit parallel) so the only thing that would limit their range (within reason*) is the amount of matter in space that might absorb/reflect the energy.
*obviously it is never going to be perfectly parallel so over interstellar distances it would diminish. Though it's worth noting that only artificial radiation in this form is ever likely to be detectable at interstellar distances. I believe our 'normal' tv and radio signals coming from point sources are only detectable out to about a light day or two. And that is also why we can still receive signals from Voyager even though its transmitter is only 23 watts.