At the risk of going on a tangent, while I love the idea of combined arms, I have heavy doubts as to an implementation of it that doesn't completely break internal consistency.
Yes, as of today, ground troopers can shoot down airplanes. But first, this is because airplanes are incredibly fragile. And then, this isn't magical tech, the airplanes themselves carry similar versions of those missiles and have the same capacity. Take that into a futuristic context where for "balance" and "TTK" reasons, spaceships can endure multiple missile volleys from other ships, and are limited to only carry so many launchers under the pretense that the hull couldn't fit more or that they'd be too big/cumbersome. From there, either (a) your ground trooper carries a similar (smaller weapon class available to a ship) launcher, and ends up shooting mosquito bites at ships, or (b) you invent a magic tech that can actually do meaningful damage to ships (up to one-shotting them) and for some reason it doesn't work when fitted to a ship hull. (a) is internally consistent tech-wise, but means that there is no meaningful handheld G2A option. (b) does provide a gamey sense of "balance" but is ridiculously contrived.
Ground-based static emplacements are another option. Here you can justify heavier warheads (they don't need to be carried around), bigger power supplies for energy weapons, or better tracking (ground tracking array) that wouldn't be practical on a ship (although the more massive the ships, the least true this becomes). But obviously, that won't cover random troopers outside of an outpost. Imho, that's still the best way to go about it. It does provide a rationale for ground troops, while not making them "viable" at the cost of a completely crazy universe tech-wise. And well, the ground troops still benefit from being tiny targets, so unless specific A2G weapons get introduced, it shouldn't be a breeze for the ship to actually hit ground targets.
I can't comment on SC as I don't play it, but taking ED as an example, look at weapon sizes. Some ships have (S)mall-only weapon emplacements. These are massive multi-ton ships and they can't even fit a (M)edium weapon. I can't see any rationale for a ground trooper carrying anything beyond S-class weapons. And good luck taking down engineered ship with your ground-based single S gun. In fact, when was the last time you took down a ship with a single M class missile? I mean, possibly with a volley of 4 thermal cascade engineered torpedoes, but apart from that? So barring magitech, hell will freeze over before ground troops are a threat to ships. Even massed-fire would require a lot of S guns to effectively become a threat. And again, if magitech is introduced with handheld launchers that can one/two/three-shot ships, well, I'll be asking Core Dynamics why they won't fit these onto every single emplacement of my dropship. For another case where this time ground-based G2A weapons are lethal, take Planetside 2. A few troopers with anti-air launchers are enough to deter air presence. But the flip side here is that PS2 aircrafts can (logically) carry similar missiles and bring the same lethality against other aircrafts.
Unless SC actually has small-sized ship-based missiles that pack a massive punch, it's likely the same issues will arise.
tl;dr: because of TTK balance, spaceships are stupidly hard, to the point where it makes no sense that a handheld weapon from a ground trooper can have a tangible effect.
