Here's my take on all this: The Guard is a military organization, tasked with protecting Equestria and everyone within it from threats foreign and domestic. They enforce the laws all over Equestria, but are concentrated in Canterlot as the center of Equestria.

They have a relatively flat organization structure with a dozen different ranks at most. They have full mobility within the ranks, mostly because of this flattened structure, however they likely use the old British method of accelerating advancement by 'buying' higher ranks. This doesn't work quite the way you might think, however. What it means is that officers actually pay for all the equipment those who report to them have. The more the officer can pay for his rank, the more men he can support. This would be why there are so many unicorns in higher ranking positions, as the show gives the impression that unicorns are in general wealthier than the other races, or at least they are in Canterlot where we've seen most of the Guard.

There are several uniforms for use in different situations, dress, day guard armour, night guard armour, undress, etc. Many fanfic-writers have proposed Day Guard and Night Guard divisions, and that's possible, but more likely it's more a case of 'which shift do you pull this week, Bob?' The armour appears to be enchanted increase uniformity in the Guard, probably a relic of prior war-like periods where such 'faceless multitudes' would increase the fearsome aspect of the military. It is likely that Luna's personal guard simply has a more extreme version of that enchantment, as we've not seen anything further around the batponies... however we have seen others with extreme variation from their 'race' norm that recalls the batponies, so I'll leave them alone for now and deal with them in a different blog topic.

The Wonderbolts are a division of the Guard used as a recruiting force amongst Pegasi. Which wouldn't be that hard to do, given the historical descriptions of Pegasi as being more militaristic in culture (Hearthswarming Eve) There are likely similar organizations for that purpose for Unicorns and Earth Ponies, but they are not as popular or as flashy as the Wonderbolts. Junior Speedsters is the youth cadet group associated with the pegasi guard. Again, likely there are similar cadet groups for the others. Junior Sparks and Junior Scouts, perhaps.

As a former Junior Speedster, and as a current Wonderbolt recruit, Rainbow Dash is likely a member of the Guard; perhaps a reserve member, rather than full-time. If the Guard do use the 'reserve' concept, then it is also possible that Applejack and Big Mac are reserve members of the Guard, simply because of their personalities.

If the Guard don’t use the ‘reserve’ concept, then Rainbow Dash needs to be already full member of the Guard in order to join the Wonderbolts, and as such that’s her day job. Since being Ponyville’s top weather pony is kind of her thing… that might mean that weather patrol is actually part of a pegasi Guard’s duties. Which would also explain why a bunch of the Wonderbolt recruits were all known weather ponies from Ponyville. They're Ponyville's Pegasi Guards who have reached whatever seniority necessary to try out for the Wonderbolts, or were dragged along by Rainbow Dash. If this is true, then the Guard’s working pegasi uniform code is very, very lax indeed or Rainbow Dash is exercising her senior rank to simply ignore the uniform code in the same way Colonel Blake in the old M*A*S*H* series did. In fact, that might be why we haven’t seen any Guard at all, Pegasi or otherwise, in Ponyville in the show, they may all be taking their cues from Rainbow Dash, and simply aren’t wearing the regulation uniforms.

The guard likely has an Intelligence Service branch, primarily focused on whatever external countries exist, like Saddle Arabia, and wherever the Griffons and Zebra normally live. Given the uptick in dangerous events ever since the Nightmare Moon incident, I would be surprised if the Guard as a group isn't contemplating pulling together Special Forces as well, possibly using the Mane 6 as a model due to their success.

They also will have stationed several more Guard in Ponyville than normal in case of emergency due to the massive target the Mane 6 all living there has painted on the town. As an alternative explanation to Rainbow Dash's attitude towards regulation uniforms; they're possibly undercover . For some reason I peg Luna as the instigator of that idea of undercover Guard, rather than Celestia. While Celestia may be a chessmaster, when she moves pieces around she doesn't really hide that she's moving stuff. She doesn't tell anyone why, but she makes no bones about her moving them. Luna strikes me as someone who's more likely to use stealth.

And finally Shining Armor was a Captain of the Guard before he became a Prince, not the Captain of the Guard. He's simply too young to have the top rank in a national military/police organization. The top rank for the Palace or Canterlot Guard, I can see, but not for all Equestria. Other cities and regions would have Captains of their own. If I was to guess to ranks, I would say that there is at least one rank above Captain; the Marshal who coordinates entire regions of Equestria, and Celestia and Luna themselves would likely have ranks of Grand Marshal. By the way, Shining may still be a Captain officially, in the same way the British Royalty all have military ranks from when they were in the service. (Yep, the Queen was a Captain, and she served as a driver and mechanic in the British armed forces during World War II before she became Queen, when she automatically became Commander-in-Chief. Can you imagine that? "Oh yeah, I was driven around by Princess Elizabeth. She had to replace the carbonator on the car once...").

I think that's about all I've got on the Guard for now. Now I need a topic for next week. Where's my twelve-sided die? It doesn't get enough love.... *roll* Okay, Elements of Harmony it is.