Spaceports on Earth

I’d say, given it’s your story, you could set them up wherever makes sense. I’d say at least one per major population centre like airports in the real world. My impression was that Vancouver was actually big enough to merge with Seattle, but could be wrong about that.

There might be a few in California. Maybe SF and LA at least?

I see no reason why there wouldn’t be. If even small colonies like Feros have spaceports then why wouldn’t Earth?

Technically not on Earth but Tamayo Point maybe? It's in Sol.

There's also probably one in Singapore, since the transport explosion there created the first biotics. I would expect that by the time the series starts spaceports have basically replaced airports and can be found in most major cities.

ME1 Timeline (Codex) refers to a shipping accident at "Singapore International Spaceport" so there was definitely one there.  The little farming colony on Eden Prime has one with two platforms, so I suspect small spaceports of that size probably exist all over (maybe even as common as small "commuter" airports), with "international" size ones inhabiting virtually all the major cities.

One I just thought of was Ashley's early rank - Gunnery Chief.  Use of the term "chief" makes me wonder if naval ranks might be more appropriate - e.g. Petty Officer, Chief Petty Officer.  Just a thought.

The lines are blurred between Navy and Marines in ME.  It seems as though the Marines are a subset of the Navy.  Ranking is therefore inconsistent.  Besides, though "chief" applies to seamen, "gunny" applies to marines.  It could go either way with her but she's stated to be a marine.  Same with Shepard (who has a Naval rank...).  Kaidan has a Marine rank.  Toombs and Jenkins are both corporals and stated to be marines.  Like I said, it might be best to ignore what's in the games and use a system that makes sense.  In some cases it could be "filling in the blanks" for ranks that maybe ought to have been present.

Serviceman 3rd Class / Private 2nd Class
Serviceman 2nd Class / Private 1st Class
Serviceman 1st Class / Corporal

NCOs

Service Chief
Gunnery Chief
Operations Chief

OFFICERS

2nd Lieutenant
1st Lieutenant
Staff Lieutenant
Lieutenant Commander
Staff Commander
Captain / Major
Rear Admiral / General
Admiral
Fleet Admiral

As I've researched more, it seems like NCOs are the promotion line from Enlisted. In that case, Ashley was near the top of the line for an NCO. With only one more rank to go, she was hardly being held back.

Kaidan and Shepard would have come in as Officers and so as 2nd Lieutenants. They would also have served longer and not moved as far up the ranks as Ash. Of course, there are inexplicably three extra ranks for Officers than Enlisted/NCOs. Scanning the USMC and USN, there are 9 ranks for Enlisted/NCO, 5 for Warrant Officers (which might be where Traynor fits in because she's a specialist, which is what WOs are) and 10 for Officers.

Actually, in looking at that, I wonder if Traynor, Joker, Adams, Daniels and Donnelly all count as something like Warrant Officers. Pressly might also have fit there until becoming an XO, in which case he would have had to be an Officer. Then again, though Adams was called "Engineer Adams" in ME1 he's a Lieutenant by ME3. That might support the idea that the unlisted "Warrant Officers" can leap to Officers, if we use Adams and Pressly as examples. Might be interesting to throw in the Warrant Officers, though I don't think it will be relevant in my story. Never know when I might write more ME fic that could use it.

I guess I'll have to suck it up and make my guys Staff Lieutenants, the rank Kaidan held in ME1, despite the 4-5 year difference in time served. Story roughly spans 2184-2186. Seems like by ME2, Kaidan had jumped up Staff Commander but must have been on the verge of being promoted since he's Major in ME3. I have to assume Ashley was Staff Lt in ME2 and then Lt Commander in ME3.

As to Shepard's rank, people have pointed out that he was listed as "CMDR" during character creation. Also, XO's are one rank below CO and so would have to be Staff Commander. This makes sense because if something happens to the Captain, the XO can step into the role.

So, back to my Alliance ranks. During ME3, there are two marines discussing deployment. It's the one where the one woman has a brother than joined Cerberus so she wants to fight Reapers instead. Her CO? "Human Sergeant". Tells me we've got another rank in the mix and maybe I can keep my guys as sergeants.

I'm not entirely clear on where the SR-2 was at the beginning of ME3. We know it was a place to which Vega and A/K could run to. Maybe they took a shuttle into the atmosphere but that seems unlikely while the Reapers were invading. It had to be on the ground, or close to it. We also saw the SR-2 land on Earth during the beam run. From the wiki, it seems like the SR-1 could also land. There has to be some kind of spaceport.