I already use light vehicles and flavor them as working like Elementals from Battletech. Compared to infantry, there's two real disadvantages:
1) Can't participate in boarding combat
2) Less benefit from fortification
As I see it, 2 is entirely fitting; someone in a big hulking suit of power armor is not going to gain as much benefit from trenches or pillboxes as someone in lighter armor that can crawl around. So 1) is the only real flavor change, and I can easily picture that as Aurora ships having narrow hallways and small bulkheads, as would make complete sense for spacecraft. Just because I like using WH40k for flavor doesn't mean all ships have vast echoing corridors.
Also if you want rare, specialized, and costly power armor you can always use gene modding for it.