Perfect Vectors don't happen:
I am not sure what kind of tech you are discussing, so I have limited my self to tech currently available - no ray guns.
In this scenario, the likelihood of shooting yourself in the back is insanely small. Space is huge, and vectors incredibly variable. If you were trying to shoot yourself, I doubt you could do it. A planet is gigantic, and it would be comparable to dropping a baseball from orbit by hand and expecting your kid in a park on the surface to be able to catch the ball.
Martial arts in zero G has already been addressed (Styles of Melee Combat developed in microgravity)The movement of objects in zero gravity is predictable, BUT the movement of people constantly thrusting and vectoring makes connecting with someone tricky, and means that pulling back an arm to swing a sword creates an equal and opposite rotation of you. A kick to an opponent pushes them away, and pushes you away as well. As hard as it would be to hit someone, once you did the fight would shift apart again. The unpredictable nature of multiple velocities and vectors means hand to hand in zero g would be incredibly frustrating. On the other hand, it might create unique opportunities - drive back opponents, use a counter weight in the opposite hand to adjust your rotation, etc. I don't see it as very effective, but I could imagine training to compensate.
In vacuum, you would find guns to still be valuable. Explosive decompression is a threat in space, but frangible bullets dissolve into tiny grains on impact with hard surfaces, dissipating the force, but still ripping intact through soft targets - like space suits and people. Even in vacuum, guns can fire, contrary to popular myth. A projectile from a gun still hits your opponent with great force to a tiny area, causing serious harm. Depending on the mass of the person and weapon firing, the effect of firing a gun could be noticeable or negligible, but if you are in a zero-G environment, you would hopefully have some form of thruster to compensate for changes in your velocity. If you don't, a gun is even more valuable - as a thruster to get you moving, not a weapon.
If you don't want to deal with changes to your velocity, there's a slightly unconventional but existing gun that performs well - the gyrojet. This is a mini-missile launcher, where the projectiles do the thrusting - little or no recoil, because the projectile has thrust, and the gun isn't exploding.
You can separate the shooting from you all together by deploying drones with projectile weapons. These miniature remote control spaceships or autonomous robots can be tiny, deploy guns, be armored, and be disposable. The people stay still while remoting around drones to kill people or other drones. Because of the nature of zero G, grenades could be fiendishly tricky but effective. A grenade with a motion-sensor can be lightly pushed in the direction of enemies and it can move fast OR slow towards opponents. Remote triggers could mean the grenade IS a drone of sorts, with it's own tiny thrusters, and it can explode on command. It could create concussion effects (if there is air) or use shrapnel. Fire can consume oxygen, and poison (gas, contact agent, etc.) can kill people, destroy air systems, or splatter toxins on surfaces, making areas temporarily uninhabitable. Even adhesives and sticky weapons would be highly effective in space, as the range of a stream of adhesive foam is almost infinite, and superglue bombs jam equipment or make opponents stick to the next wall the touch, all without the pesky interference of gravity.