I dont think the answer to figuring out alien rank systems is to find equivalents.. obviously they could skip or add lots of grades depending on their tradition, or ideas of what makes a force effective.

Heres the romulan way ;-)
Uhlan -- referenced by Neral, who was once a 'lowly uhlan in the Romulan guard' Its actually a Terran word (brr.. i just called us Terrans)defined by websters as 'any of a body of Prussian light cavalry, org. modeled after tatar lancers. Non-com i assume, most likely infantry rank not associated with fleet navy.
Sub-Lieutenant -- Jarok's alias was 'Sublieutenant Setol'
Lieutenant -- Assumes that a sub-lieutenant would have a one higher grade
Sub-Centurion -- not sure if weve actually heard this one, or if its just stuck in my head from Diane Duane's rihannssu. Its a little wierd, meaning 'guy under the guy who commands 100 guys'
Centurion -- from 'Balance of Terror' The roman centurion was so named because he commanded 100 men. given the small size of the BOP, it seems unlikely the centurions aboard those had 100 troops under their command (except maybe in Diane Carey's mothership scenario for pre-cloaking Roms)
Major -- Rank used in the Tal Shiar. definitely not associated with naval, indicates different branches
Sub-Commander -- from 'Enterprise Incident' Typical XO or even ships commander of lower rank than senior commander. (as seen in later shows where subcommander is the person in charge of even a big warbird)
Commander -- starship commander (Balance of Terror) perhaps a little higher than post captain in fleet, considering that subcommanders command a lot more than presumably equivalent fleet commanders
General -- from 'Redemption' possibly an ground rank not associated with Romulan navy.
Admiral -- from 'Defector' Romulan flag officer.

I guess that we could take "uhlan" as being a direct application of the Earth term rather than an alien word of accidentally identical pronunciation. But one can be "lowly uhlan" early on and "high-ranking uhlan" after a round of promotions - as you say, the Uhlans were not a specific rank but rather a "service". One could hold a rate in uhlans, or in combat engineers, or in anti-aircraft artillery, and the rate would not be directly tied to the type of service.

(Of course, historically these "special forces" also had special rank systems or rank names - a 1st Lt. in cavalry would be a "Cornet", if you want a truly corny example... But in general, "standard" rank systems were applied, with minor modifications.)

So our Romulan could have been a lowly Uhlan in the Praetorial Horse Guard at first, of the rank of Sub-Lieutenant or perhaps the rate of Antedecurion, then been either promoted to a "high" Uhlan or transferred to other duties. I don't think the word should be taken as indicating a rank, not with all the other Romulan ranks being simple English descriptive terms.

And the rank system with sub-lieutenants and so forth could be separate from the rank system with pseudo-Roman ranks, really. Centurion (and of course decurion etc.) might come from a special Praetorian Guard, with representatives aboard select ships where people with "naval" ranks serve in greater numbers. The role of the old centurion in "BoT" seemed to be one of observer.

To be frank, I'd be happiest with a *threefold* Romulan organization. First, a space navy with naval-style ranks, that is, with lots of "commanders" and "lieutenants" and "admirals" in it. Second, a separate ground service, of which Tal'Shiar is part, with army-style ranks. And third, a special Praetor's Guard with pseudo-Roman ranks, perhaps originally the Praetor's politruk force but currently subjugated by the more aggressive Tal'Shiar politruks.

The UT would have been programmed with knowledge of the threefold division in mind. It would use direct US Army equivalents for the ground force, since those would be applicable. The navy (or ex-air force) could be more complex and byzantine, playing a greater role in Romulan history, so the UT would have to go creative and invent in-between ranks with "sub" -prefices. And the Praetor's Guard would be a historical relic to begin with, so the UT would be happy to play with pseudo-Roman names, to go with the pseudo-Roman UT designation for the races and the planets.

With Klingons, direct USN and US Army models could be used by the UT. With Cardassians, the system would be so utterly different that no US ranks could be applied - a Gul would be a Captain and an Admiral and a Governor all at once and more, a Glinn would be anything from Ensign to Commander plus Aide and Secondant and Yeoman all in one, and the UT would explode in a puff of smoke in an attempt to properly translate the complex concepts.

If the ranks had simple equivalents in the USN system, or even in the English language, the UT ought to translate them as such.

If the commander of a Cardassian ship is considered a Gul instead of a Captain, then he or she is probably something other than a captain by rank and position. Indeed, Guls seem to come in many hierarchial levels and play a role in "civilian" politics as well.

A Ferengi Daimon in turn is probably not only a captain, but a businessman as well, in charge of profitmaking as well as of flying his vessel around.

At least I hope all or most of the alien ranks and "ranks" can be explained this way. It would be pretty annoying to have to say "The UT malfunctioned again and forgot to translate" every time...