Honestly, I think you'd be better off either building a space station, or stationing them near a planet. If it's the latter, a location sub wouldn't be needed, just make sure the Navy knows 'this is where we all hang out'.
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It's a feel thing. How much effort you put in to crafting a story towards the object/species/location/etc. In general you want at least ten posts, but if you skimp on them your judge'll find out and probably request more development.
Just write a thread like you would any other and put in some direction towards buildling your submission. The first two hundred posts of the galactic alliance's flagship thread was a fundraiser ball social thread with the entire board invited. Dev threads don't have to be "I hit my hot blade with my hammer and give it shape" for ten posts, just something to show that creating/obtaining thing x is part of your character's story.
Of course you can. Use of Codex and Factory is not mandatory (until it is/until someone reports you).
Which is to say, you don't have to submit a location to Codex in order to use it. Many people will create a location through their threads and submit it for ease of reference to the Codex later on, using those threads as development.