The GameInformer article thread

Don't forget Vega being offered entrance into the N7 programme.

Also, I forget, in the Alliance, how many ranks is Major above Lieutenant? 2? It's worth noting, though, that Kaidan only outranks Shepard on away-missions, since Shep's still the captain of the ship, regardless of rank.

You could probably excuse Garrus, too, by virtue of him only having that rank on account of tons of other people in the line dying.

If you take that route to the logical extreme, you're not going to have very many squadmates. Bioware doesn't make them all plot-important/critical, which is probably a good thing considering it would probably introduce many contrivances into the main plot in order to make all of them critical in the first place.

Besides, as far as I'm concerned, Garrus or any other squadmate does have reasons to be there. They're skilled and depending on your character, are Shepard's friend.

You might want to cut Tali a bit of slack. I'm sure you've heard of someone freezing in a tense situation. Sometimes in disbelief, sometimes for other reasons. Fight, flight, or freeze. Not to mention she's barely an adult and is not a hardened soldier by any means, nor is she supposed to be.

But all of that is ignoring the real reasons Bioware makes characters do or not do things in cutscenes, and sometimes they're not reflective of a character's personality, but merely accommodating an expedient scene. Or just sloppy. It happens.

In the real world Major and Lt. Commander are basically the same rank, just for two different branches of service. Major is the Army or Marine equivalent of a Navy Lt. Commander. In the Alliance rank structure I believe Major is two ranks above Lt. Commander. If I remember right, an Alliance Major was the groundside equivalent of naval Captain, with Staff Commander in between. 

In retrospect I wonder if maybe the writers forgot they made Major a much higher rank in the Alliance than it is in the real world. If an Alliance Major was the same as the real world rank, Kaidan and Shepard would be basically be the same rank, but Shepard is still the senior officer by virtue of having been promoted into it earlier. 

That is at least assuming that there is no dialogue acknowledging that Kaidan outranks Shepard in ME3. I can't recall now.

That's incorrect actually.

A Spectre is outside the Alliance chain-of-command. In order for a Spectre to outrank any of the military officers, Spectres would have to be part of the organizational hierarchy and the chain-of-command, rather than being separate from it.

For example in ME1 Nihlus is not in command of the Normandy, Captain Anderson is. Spectre status alone isn't enough to confer the authority to command an Alliance warship to Nihlus. He is merely a passenger, albeit an important one. Anderson, and later Shepard, command Alliance warships because they're officers in the Alliance navy. The authority to command the vessel is conferred from their position within the Alliance's organizational hierarchy. That's as true for Shepard as it was for Anderson. Remember it is Udina (an Alliance politician) who arranges for Shepard to command the Normandy after Anderson is forced to step down. All Spectre status grants is the ability to operate independently from Alliance command. Shepard determines where the Normandy goes and what it does, rather than some Admiral. The crew however answers to Shepard because he or she is an Alliance officer.

There have been real world equivalents. Many of the of the OSS and SOE operatives  that were parachuted in occupied France during the Second World War held simultaneous positions as both members of civilian intelligence agencies, and officers in their respective country's armed forces. Any authority to command military personnel would have come from their status as military officers, not their status as intelligence operatives.

Spectres, or at least those from military backgrounds like Shepard, are pretty much the same deal as those OSS or SOE operatives, except with some Judge Dredd thrown in.