Seeing how if we use our warp drives to 'colonize' space, other creatures would have far more to fear from us than we from them (think Independence day with roles reversed). If a space faring species encounters non-space faring species, the one with warp is probably (not necessary) in the better position. Even without the weapons, FTL drive implies a magnitudes greater technology than anything a non-FTL race would have. In fact atm we might be the most dangerous creature out there, so in future (if we get that far) I probably see human species branching out into several subspecies and then waging endless war against each other.

Halo is a work of fiction, not a real life based drama. We'll probably be lucky if we find anything more interesting than a microbe. First encounter will probably be more like Solaris than Halo. We'll most likely find an organism that we can't even perceive as one.

So no, we don't need guns in space. Best weapon in space is Newton's second law (i.e. Kinetic bombardment). Cheap and easy to set up, hard to defend if your target is in a gravitation well.