Ah, but the Americans in the Revolution DID have a base uniform...
Continental soldiers wore blue coats, white pants, and the blue coats had red or white (or sometimes other) trim dependant upon station, rank, etc...
The Rebels of the CSA also had a "Standard" uniform, though variations in the confederate uniform were quite common... Then again, they were common for soldiers in the Union as well, depending upon regiment... Union Sharpshooters for instance wore green uniforms. It was also a time a uniform's "style" sometimes outweighed it's function though, and thus that nullifies the use of various uniforms from the North for this conversation's purposes... Star Wars seems to rely very little on style, and 100% on function it seems.
I've read the "rebels wouldn't wear set uniforms" argument at the Tech Commentaries, and to me that's thinking entirely too much "earth", and not enough "Galactic"...
Lucas described Star Wars as a "Million Million worlds", which I'd wager is a LOTTA worlds.
Of those million million, how many are loyal to the Empire? Let's say 3/4, or maybe 4/5 would be more accurate? Maybe even 9/10... Who knows really? But how many are loyal to the Alliance? 9/10 of a million million worlds I bet is a LOT of worlds still...
Out of the planets NOT loyal to the Empire though, would they not have the manufacturing ability to make a "basic" uniform the Alliance considered a "Standard"? I'd think Earth alone would be capable of supplying the uniforms of what would be the majority of the Alliance military... Maybe not, but what about 2 Earth-like planets? Or 3? What if there's one planet dedicated to this task?
But, let's put it in some Earth terms then anyway.
There's maybe a "Standard" uniform for the Alliance Fleet (Thus the Fleet Trooper). I pick this uniform because it seems the most abundant, and it does not look "professional". IE: it's not taylored well, and there's something haphazard about it... Most of the soldiers look clumsy, young, and it has a "ragtag" feel to it while still keeping a distinct uniform for the troops to call out their friends from their foes in a battle.
It's definately not shaping up to be the taylored and impressive looking uniform that a planet's own security force might have though, like Amidalla's troops on Naboo who wear quite nice uniforms. It's also not impressive and "expensive" looking like the Empire's duds are.
That to me implies that the RFT uniform is that of an Alliance Army Regular... Or in this case, an Alliance Fleet Trooper. It's cheap, but it has function and some level of form. It could be cranked out en masse, at low cost, and will supply structure to a main military enlisted force.
To take some rebel uniform continuity further on-screen, we can see rebel petty officers and crew at the ceremony on Yavin. I've counted quite a few variances in the uniforms at Yavin, but they're structured in a way, and it's known that a military advisor was on-set for ANH and ESB (Not known if the same person was there for ROTJ). Thus the uniforms are put there intentionally to show some level of military structure at the final ceremony.
Similar, or identical uniforms can be seen manning various terminals and stations aboard Home One as well, which implies some level of uniform carry over between ANH and ROTJ... They even have the same odd yellow emblem on their tan/brown hats on Home One as they do standing while Luke/Han get their medals.
But, taking the "earth" theory even a bit further in support of structured uniforms...
We know the Alliance is made up of more than a handful of human planets... We see "uniforms" of other alien races represented. For instance, the Dresellians have a uniform distinct to their species, as do the Tibrians... One can argue the Mon Calamari do as well, but we seem to only see Mon Calamari "officers" so it's possible that their uniform carries over to human crew or other crew.
Either way though, there's some uniform variety, but it's possible the Alliance works something akin to rebelling forces in Earth's history.
For instance, let's say the RFT and subsequent known uniforms associated with the RFT's (Like Captain Antilles', or Dodonna's) are the "basic" uniform of the Alliance "Regulars" like I've theorized... Essentially the uniforms you see at the Yavin Ceremony.
These are, for an analogy, the "U.S. Continental army".
But the Dresellians have been fighting a resistance movement on their home planet against the Empire. They're openly rebellious, and are allies to the Rebel Alliance, but possibly act independantly... They maintain their uniforms though, for a variety of reasons (Maybe they choose not to commit to being army regulars, like the militia/minute men did in the American REvolution, who were "free" to leave at almost any time due to their status of not being part of the regular army).
IE: The the Dresellians, and possibly other races = non-army regulars, or militia/minute-men type of troops...
The Continental army will take up the brunt of the fighting in open conflict... Militia may be right alongside them, or maybe are fighting on their own, closer to their homes... Some Dressellians (or other species) may be within the Continental army itself, and wear the uniform of it... Some may not.
So while ragtag in a way, the Alliance troops still have a main contingency of soldiers spread throughout various fleets and leadership groups around the galaxy.
They would also have various volunteer, support, militia, and guerilla pockets throughout the galaxy, as well as possible reserves, "national" guard, etc., forces at their disposeal... Each of these groups may wear a variety of unique and different uniforms... Some may be planetary security forces, or some may just be make-shift camoflaged uniforms made for their specific planet.
Another analogy, and one more close to our own time, would be the Vietnam War. The Viet Cong were guerilla fighters with no "set" uniform... Some dressed like civilians and struck from the streets of Sai Gon, some were in the classic black pajamas, sandals, and rice hat clothing, but that's hardly a "uniform"... It wasn't much different than what a civilian may wear, and was more for function over form.
Then though were the NVA... These were the "Regulars", and they wore a khaki uniform, very simple and basic, and a light expedition helmet. They were also better supplied, and better supported/reinforced troops. Whether their tactics worked better is up to debate though all together, but they were a noticeably more uniform force fighting for the same cause, and alongside the Viet Cong and it's varied cells.
To me, while putting Earth theory into practice it sometimes isn't always as logical to think that way when you're getting answers to a Galactic Question. Sometimes it works though, and sometimes it doesn't... Sometimes it just depends how you're looking at things (Like WHAT rebelling army you'd compare the Alliance to, for instance).
My main theory is that in all the rebelling planets, which while fewer than the Empire by far they would still number into the thousands possibly, the Alliance would LIKELY
have the ability to make some form of standardized uniform/system for their military.
When fighting would take place on this galactic scale, a uniformed structure would be necessary just to maintain order I believe.
That uniform structure would NOT exclude any outside help of course, and just like the Zouaves of New York in the civil war, while they looked like no other regiment in the Union's army, their help was never turned away because they were passionate volunteers... Even though they wore sneaky pierre hats with little tassles and bright red pants (Talk about an easy target for the rebs!).