Combat is a complex thing, even for a species that's developed never knowing anything but. Throw that species into a weightless environment, and you have even more of a problem.
First, a look at conditions to be considered.
Type 1 is 0g, 0 atmosphere. This is the true realm of the Interstellar Combatant. Found on derelict ships, broken-open space stations, asteroid mining facilities, and anywhere utterly inhospitable to human habitation. Movement must be handled by kicking off from something, the use of a tether/grapple, or rockets, and all motion requires some manner of delta v to stop (a wall, or rocket boost in the opposite direction).
Type 2 is 0g, with atmosphere. Atmosphere may not be breathable, but it's present. This is likely to be found on still sealed ships and stations that either lack artificial gravity or have had it damaged. The resistance provided gives options for movement beyond rocket boosters or tethers once the feet are away from a wall.
Type 3 is nonzero gravity, regardless of atmosphere. Even at lower or higher gravity, most of what we know about combat works with some variations, although some tactics may be possible or unworkable due to gravitational factors.
Type 1 combat requires heavy use of tethers. Floating in space leaves one incredibly exposed, and due to the nature of 0g, it's essentially impossible to move or dodge. Magnet boots or something like gecko-grip boots permit a semblance of walking without tethers, but aren't to be trusted in cases where floating debris may prohibit a good seal, or sensitive equipment leaves no adequate safe place for the use of magnet boots. Tethers would likely be mounted to either side of the hips, similar to how they are with modern day astronauts. Unlike modern astronauts, they would likely use a spooling mechanism with auto-retraction, allowing the interstellar combatant a semblance of mobility between deployed tethers. Combined with a remote detachment capability, they can be used to leapfrog while retaining something like a braced position. Other tethers would be some manner of grapple, used to pull a combatant to an object, either in case of having lost their attachment and being floating away, or to change directions after a launch.
Type 2 combat permits the use of jets, fans, and other mobility aids that require atmosphere to press against, with varying effectiveness based on fluid density. All type 1 utilities remain viable, with the consideration that motion will now come to an end due to air resistance. Boosted jumps and greatly improved control in general can be had, without increasing fuel carried for chemical rockets or similar.
Type 3 combat may benefit from the use of several Type 1 and Type 2 mobility enhancements. Jets or boosters may be used to enhance mobility in low G worlds where the human body cannot produce a comfortable, natural gait, or permit bypassing of obstacles in various manners.
Specialty Tactics: Type 1 and 2 environments permit, if not require, the use of modified portable cover. Dome shaped, heavily armored, and equipped with jets, fans, or rockets, it serves as a means of crossing empty space where necessary without being completely exposed. Makeshift versions can be had from any solid object that can be freed and given some momentum to serve as cover. Tethers may be used in conjunction with corners to permit SWAT style entry tactics to move around obstacles quickly.