Can you mount medium weapons on light vehicles? I don't think that was an option for me.
Yes you can. My Mech Inf battalions usually have CAP, MAV, and MAA vehicles supporting the infantry.
I have a mechanized battalion made up entirely of Medium Vehicles with Heavy Anti-Personal to help with breaking through and mopping up infantry. Its not a MAC, but I thought it would be best since I can then have more IFVs and can fire more shots. NPRs favor infantry-heavy armies I understand, and ideally most of my armor-piecing will come from orbital air support, thought I understand ground fighters still aren't working properly.
At equal tech, HCAP doesn't give you a lot that CAP doesn't but it will help you penetrate light vehicles and static elements. MAC is more effective against both of these though but as you note is quite heavy. If the NPR has higher armor tech then HCAP can help cover that gap for you.
Ground attack fighters I believe work fine, they just suffer from being targeted by both STOs and AA which makes them very vulnerable unless you're willing to orbital-bombard the STOs first. Even then AA is quite effective.
I hope I won't eventually be forced to field infantry all kitted out in power armor. I keep that for my special forces for RP purposes, along with Advanced Genetic Enhancement. Offensive infantry can get Improved Genetic Enhancement tho.
Not forced, and if you don't want to then light vehicles remain very useful. However if you use infantry offensively, armor + HP enhancement is optimal as you want to maximize combat power per ton for an offensive formation (assuming drop transport capacity is the limiting factor). On defense then cheaper units are good to maximize combat power per unit cost.
I'd love to have larger formations of Marines but I can't fit them into a fighter or FAC sized ship and I really don't want to build a special ship just for deploying marines. I wish you could split a formation up across multiple ships in the same task force.
One option to get 500-ton formations is to use a FAC-sized ship with a boosted engine and low fuel range to get enough speed, and deploy them from a carrier. Otherwise, 250-ton units are fine as long as you commit to using multiple of them for most boarding attempts. The important thing is to have sufficient total tonnage and not rely on just 250 tons of marines to capture entire enemy fleets.
I don't think unarmored infantry should generally be used in attacks except for emergencies, and on defense the light vehicles can't entrench to the same level as the infantry, so I think the motorized company is kind of working against itself in both roles. The foot infantry company could be a good cheap versatile unit for light defense and policing. If you want a unit you can put on front line attack to go with the battalion, I'd suggest using one with all light vehicles, but I think you might be better off building a separate battalion entirely for attacking. Your battalions are small enough that they can specialize and shouldn't be operating independently, so they don't need to cover all tasks on their own. Maybe you could build something like a regiment of 2 infantry battalions and a light vehicle battalion with some support units and that could hold down a minor colony by itself or be grouped into brigades and divisions on larger ones.
Infantry works best in a combined-arms role i.e. if you also bring tank/armored battalions into the attack force. They will draw anti-tank fire away from the vehicles to reduce the enemy's tonnage kill rate somewhat, and are also fairly effective at killing enemy infantry. So it can be useful on the attack as well even if infantry by itself is not a strong attacking unit.
Defending infantry are mostly protected by fortification bonuses and sheer numbers and not armor, and vehicles are usually a better choice when attacking, so I think it'd be kind of a waste to use armored infantry in entirely ground-based armies under most circumstances. Power armor is about using a limited tonnage more efficiently and unless you're boarding or dropping from orbit under fire you can move as many tons of troops as you like pretty easily.
All things being equal, armored infantry would perform substantially better even on the defensive. However on the defensive you want to maximize based on cost which means unarmored infantry gives you more boots on the ground. On offense you usually maximize for tonnage efficiency due to limits on transport capacity, thus armor becomes more important.
I imagine you can get away with using small units of marines to board ships if you make sure to land multiple units at the same time. There's a fair chance that a given boarding shuttle won't successfully land on its target anyway so redundancy is good. You could build a light assault carrier in the 10-20 thousand ton range with a number of FACs capable of landing several 250t marine boarding parties at once, with some space for more troops onboard. Or maybe you could get the same capability by building a cruiser squadron with hangar space for one boarding shuttle in each ship that could conduct boarding attacks with the same number of troops - then all your ships would have some marines on board to defend them, too.
All excellent ideas. Note that landing is on a per-element basis not a per-shuttle basis, but multiple shuttles does still improve the chances of landing enough troops to take the ship which is the idea here.
Marines with PWL can be squeezed very tightly into boarding shuttles and can still fight effectively against crew (unless they have way better tech than you, but in that case you're probably not capturing their ships anyway), so unless you expect to face enemy marines they're probably a better choice than PW or PWI. IIRC CAP marines still put out more shots per ton but PWL doesn't use much supply and can soak more hits, so they might be good to send in first against a very large crew that'll fight a long time. I haven't run any numbers on it though. Don't forget to bring supply with your marines, too.
PWL is the second-most effective unit against equal tech (after CAP), but if the enemy has even one tech level of armor more than you it will fail badly. CAP is largely immune to this for boarding combat. You do save on supplies with PWL but I'm not sure that's a major concern for marine forces as their overall small size means supplying them is not too expensive. Maybe logistically difficult.
Fights against crews of a higher tech level are not as uncommon as you might think, notably if you board
Precursor ships you can expect this to be the case unless you're waiting for tech parity to start that fight - I for one don't wait.