I don't think adding a powerful, hi-tech military force (that doesn't exist in lore) to the game is a good idea. Most free companies get away with having magitek weaponry because they're just a handful of adventurers that don't mean much in the grand scheme of things. But playing your FC as a big, heavily armed military force is a different matter entirely - this is something that would change the global balance of power in Eorzea. You're pretty much forcing others to either agree that yes, your military force is (or is going to be) a major global player, or to ignore you. And most people will probably go with the latter.
As for playing Garleans:
Garlean refugee who's loyal to Eorzea - a very viable concept. Hell, I'm playing such a character. You probably shouldn't ba a pureblood Garlean, though, just someone from a land they conquered.
Garlean character who secretly supports the empire - same as other villain characters. Which is to say, thread carefully, because most people don't know how to play villains well.
Garlean character who openly supports the empire - no. Just don't. How are you even still alive?
As for playing Garleans:
Garlean refugee who's loyal to Eorzea - a very viable concept. Hell, I'm playing such a character. You probably shouldn't ba a pureblood Garlean, though, just someone from a land they conquered.
Garlean character who secretly supports the empire - same as other villain characters. Which is to say, thread carefully, because most people don't know how to play villains well.
Garlean character who openly supports the empire - no. Just don't. How are you even still alive?