Grounded vs. Flight vs. Space is a matter of the technology level or rules of the fictional universe. The lower the technology level, the more grounded you are, like Patlabor, Votoms or the show last year, Argevollen. The higher the technology level, the mech should match that.
For example, the recent excellent Aldnoah Zero which concluded early this year. The mechs are primarily ground combat only, but they also have space versions. Space is weightless, put boosters and jetpacks, the mechs would be "flying" in space fast and easy. Its only becomes a matter of navigation and control. If I put a Battlemech in space with jumpjets, adapt the navigation and stabilization systems to space, space proof the living compartment, it will be "flying" in space like Gundams. In fact, it should be attainable with the level of technology in the Battletech universe. And realistically, the difference of weights in planets should affect the mobility, so its not impossible for a jumpjet Highlander to be boosting around like an Armored Core mech on a low gravity moon or asteroid. In fact, a jumpjet Highlander is going to be pretty fast, and jumping at great distances if its moving on our moon.
Without gravity and atmosphere, a Battlemech in space can shoot its medium lasers even at great distances, with infinity potential. There would be no range limits to LRMs, SRMs, and even the AC20s, and everything will fly at far greater speeds since there is no aerodynamic drag. The important thing is to adjust the fire control systems for space use.
Flying and fighting on Earth's gravity and with the density of Earth's atmosphere, that is something though. If I were making a sci fi mech universe, I would have mechs adapted specially for fighting in space, then for fighting in M class planets. I will leave aerodynamic fighters for atmospheric combat. Should note that in space, a mech or a fighter would be equals; the mech has no aerodynamic drag to disadvantage its frame, and it has the advantage of being able to achieve a full spherical coverage with its main weapons.
Another thing about fictional mech universes, if the antagonist is another human faction, the technology levels are going to be lower, generally more equal with each other. Some might like to call it "realistic" but its a useless term for science fiction. In SF, realism is not important --- technology consistency to the rules you laid in that universe is, whether those rules are based on pseudo science, science or even magic. That one thing that matters is consistent. If your fictional universe has mechs powered by magic, then they have to be all powered by magic, you cannot suddenly turn around by introducing a mech that is powered by something else.
In a universe where the antagonists are non human, especially alien, the technology rules tend to be much higher, especially when space combat is involved, when the era can be over well over a thousand years from the present time. In Knights of Sidonia, its already over a thousand years since the Solar System was destroyed. There is no specific date for the Solar System's destruction but it should have taken place at a time when Humanity has reached the level of being able to carve asteroids and turn them into colony ships. Which by itself is a pretty advanced level of technology. When the antagonists are non human, and showing much greater capabilities, you would need to advance your technology to the level of being able to fight them, and so the driving forces for advancing technology are much greater. This does not mean that that there won't be "realistic" mechs typical of Man vs. Man mech universes to appear in Man vs. Alien universes, but usually the portrayal of "realistic" mechs in Man vs. Alien mech universe, such mechs ended up being fodder and easily destroyed, causing a technology escalation and race to develop super mechs to match those aliens in the first place.
Edited by Anjian, 29 June 2015 - 07:39 PM.