And Ryan was evidentially the XO of a full General and one of the Joint Chiefs of Staff. You really think someone just one paygrade up from a mere Lieutenant would get that job? I once worked as a civilian clerk in the RHQ of a mere artillery regiment and I can tell you that the CO, who was a Lieutenant Colonel had two adjutants, one a Captain, one a Major and another Major as his 2IC.
Do you really think a General's flagship would be left under the command of anyone BELOW the rank of Captain in the General's absence? Of course not, that's insane.
From what was mentione of Major Ryan, this makes much more sense.
http://en.wikipedia.org/wiki/Aide-de-camp#United_States
There may have been other generals, captains, or other officers that worked with Hague or that served on the Alexander, but Major Ryan was more than likely Hague's aide-de-camp. I would guess that when they broke away from Earth, since Major Ryan was clearly a fairly high ranking officer(however high a major is it is clearly a rank you have to be experienced to get) and as he was Hague's aide-de-camp, I would think those two facts would give him authority in and of itself.
As for him wearing a navy blue uniform, this is actually quite common in the US Marine Corps. The US Marines do not have doctors, nurses, or chaplains. All medical and religious support staff for the US Marine Corps are commissioned officers or enlisted personnel from the US Navy. That's why a lot of times you hear about US Navy Hospital Corpsmen attached to Marine rifle squads or other units for medical support. Even though they are US Navy personnel, as they are attached to Marine units, they often wear US Navy uniforms, though they keep US Navy insignia.
http://en.wikipedia.org/wiki/Unifor...aval_personnel_attached_to_Marine_Corps_units
http://en.wikipedia.org/wiki/File:Personnel_man_the_rails_on_USS_Blue_Ridge_(LCC_19).jpg
Marine uniform, but still US Navy rank insignia and titles. The British Royal Navy and British Marines do this as well from my understanding, so this isn't something that only the US does.
Also, this might be revelant for some ranks.
http://www.slasims.com/resources/rankconversion.html
http://www.b5tech.com/oldb5tech/misctech/earthforcerank/earthforcerank.html
For the navy named ranks, I know both sites are correct for Junior Grade Lieutenant through Captain, as well as Major.
Also, if your trying to figure out where B5 and Earth security ties in, I think it would be a good idea to view Security as similar to the US Coast Guard.
http://en.wikipedia.org/wiki/United_States_Coast_Guard
While the US Coast Guard is a uniformed service and uses the same ranks as the Navy, unlike the four main branches of the service, the Coast Guard is not considered an official part of the military. Because of this, in peace time, they have law enforcement and arrest powers for arresting drug dealers, people that break environmental or other federal laws, and can arrest people or carry a firearm when off duty, something that the US Navy, Marines, Army, and Coast Guard are not allowed to due because of federal law.
http://en.wikipedia.org/wiki/United_States_Coast_Guard#Authority_as_a_law_enforcement_agency
This would allow them to use army/marine ranks and titles as well as uniforms, but would explain why they are enforcing the law on a civilian populated space station.