Ok so speaking honestly, considering every launch into space is still a crap your pants event where thousands of things can go wrong, the notion of conducting warfare in space seems totally silly.
However, in order to minimize the reactive force recoil would have on anyone, it seems a low recoiling, but highly destructive fast round would perhaps be a better choice. I'm thinking the 5.7x28 or even the FK-BRNO round that's supposed to hit 2000 fps from a pistol. However it seems the perceived recoil will be irrelevant compared to the thrust generated by the expanding gasses. So I think you're screwed either way, whether it be big and heavy or light and fast. Seems light and slow would be the only effective mitigation. And that's not what you want. So to be honest, I think any gun chosen will be pretty useless unless other measures are taken to keep a shooter in place. Of course it's been a long time since high school physics.We aren't going to see armed incursions of enemy space stations and such nonsense.
I think what we will see is enemy satellites being targeted, along with orbit to surface ordinance delivery. Maybe even targeted supply drops to soldiers on the ground.
What makes a lot more sense to me is drones capable of leaving a mother craft of some kind, and delivering enough of a barrage of projectiles to disable enemy vehicles or satellites. So I'd say pick something narrow and fast again, that maximizes capacity on those little buggers, give them a thrust capability tuned to deal with the recoil impulse of the guns, and choose a projectile type (monolithics) that can penetrate the hull or outer armor of enemy equipment. It will escalate of course, but I think to start, small and fast drones, carrying small caliber high velocity projectiles with a compensation mechanism is where we are going.
Sidearms? I really doubt there'd be a need. I mean wouldn't gunfire of any kind inside a spacecraft compromise the hull and basically lead to an explosive decompression?
