For those interested, this is one of those things I've personally picked apart in my normal meticulous/anal manner, in hopes to create a database of the exact uniform structure to the Rebel Alliance.
Their uniforms include:
1) Brown-Green (More brown) uniforms... Fairly simple, long/straight-leg pants over boots or shoes, simple button-down (ill fitting) tunic, and open "cargo" pockets on the pants. No "ornaments" like flashy buttons or chevrons. Tan "ballcap" style hat with yellow/red emblem of some sort (diamond-shaped?) in the center. They appear to be a low-level officer of some sort... Maybe akin to black tuniced Imperials in the films? Large "Star Wars-style" belt included, either black or brown. (most common uniform style in the ceremony scene... MAY have hat variations where only a seeming "commander" type has the emblem, I forget and don't have images handy to check).
2) Brown-Green (More brown) Uniforms... A more complex uniform with pockets, chevrons, dark brown or black buttons. A better fitting uniform, more in-line with an officer's uniform. Long-brimmed tan hat, and an "ascot" or scarf of some sort at the collar. A large SW Style belt included again, either black or brown. NO emblem on hat. (Officer WITH the medals in a case is one of these officers)
3) (POSSIBLE) Tan-ish uniform... Only ONE guy at the whole set-up it seemed. Was standing on the opposite side of the viewscreen to Dodonna when he was going over the battleplans. Only viewable in widescreen and edited shots. His uniform's similar to #2's uniform, but lighter.
4) Rebel Technician... Tan/light grey jumpsuit. High collared, wears a RFT helmet WITHOUT neckguard. In ROTJ they had their pantlegs tucked into the boots, and in ANH they had them over the boots. It's assumed it's not significant. Seen wearing headsets or no headgear at all also.
5) Rebel Fleet Officer... Captain Antilles uniform... The only known variation to the uniform is the long trench-coat, which may have just been an added accessory that Dodonna just happened to be wearing to keep equipment and such in the pockets... Something akin to a Doctor's smock or something? The coat didn't seem to be of any significance though, and all officers with the tan uniform may have that coat for all we know.
6) Fleet Trooper... Standard Non-Commissioned troops... Blue shirts, black tech vests, grey cargo pants, lace-less combat boots, white domed helmet WITH neckguard and black visor... We all know these guys right? They suck... Well, Hasbro's figures of him suck anyway.
7) Fleet Tech Officer... A C/O of a Tech unit perhaps? In ANH the "Tech" that talks to the fighters attacking the Death Star via a headset wears a Rebel FLeet Trooper-style shirt (same cut/collar) that's the color of a Tech's uniform. Also has on pants that match the same color and a brown belt. Definitely a unique guy, and seems more important in the Yavin war room as he's in direct contact with the fighters. Also seen in the Briefing scenes standing/observing.
Rebel Pilots... Tons of em at Yavin, like possibly hundreds. Most with powder blue emblems and black visors.
9) TONS of EU stuff to sift through... When I do my lil compelation of this material I may take "artistic liberties" with EU material though, as it's hardly coordinated.
10) ROTJ uniforms don't change much in some cases, then there's unique officers like Mon Calamari, and Madine's group, but they could be separate branches of Alliance military...
I count Madine's uniform style as that of the Alliance Spec. Forces (Which EU coincides with on his background anyway), and that they're almost a separate branch of the military from "Alliance Fleet Operations". This would go with the theory that the Alliance tries not to openly engage the Empire in open combat because they get trounced easily.
Basically, if need be the Troopers of the Fleet are deployed in situations, and they perform the "brunt" of the work being done fot he Alliance military, but the more active group in combat is Spec. Forces and they are a separate entity of the Fleet Forces because of that.
Like our own military, the Marines go into HUGE situations like our attack on Iraq, but the Spec. Forces are constantly coming and going from various parts of the world for various combat situations.
I list Dresellians and Tibrians as separate species not willing to devote themselves to the main body of the Rebel Alliance, but still allied with that force... Consider it like having Canadian troops standing next to U.S. troops at a briefing for D-Day... The Canuck guys will look different, and aren't a part of the U.S. military which is a larger force, but they're fighting the same cause...
OK, that's a lotta crap I just posted. Sorry. If I can ever get scans of any of these uniforms that are clear, I'll be more than willing to post them, or send them over to someone who can post them rather.