Depends on the Special Forces unit. The following is mainly from the GURPS: SpecOps book. You can find a lot more data from the SpecialOperations.Com website. I'm guessing you'll be most interested in the
US Armed Forces Special Operations Units section.
The US Army Special Forces (the "Green Berets")Pperational forces consist of Operational Detachment Alphas, which have 2 Officers and 10 NCOs:
Detachment Commander (Captain)
Executive Officer (Warrant Officer)
Team Operations NCO (Master Sergeant)
Intelligence and Operations NCO (Seargeant 1st Class)
Weapons NCO (SFC)
Medical Specialist (SFC)
Engineer NCO (SFC)
Communications NCO (SFC)
Assistant Weapons NCO (Sergeant or Staff Sergeant)
Assistant Medical Specialist (SGT or SSG)
Assistant Engineer NCO (SGT or SSG)
Assistant Communications NCO (SGT or SSG)
The next highest unit structure in Special Forces is the Company (probably lead by a Major?), then the Battalion (Lieutenant Colonel?) and then the Special Forces Groups (Brigade-sized, lead by Colonels maybe?).
1st SFOD-DeltaThree operational squadrons, each about 75 soldiers, divided into 4-6 man teams. All members are SFCs or higher. Dunno about squadron leader grades, but the one who lead the C-Squadron in Mogadishu was, apparently, a Lieutenant Colonel.
US Navy SEALsCouldn't confirm grades. [Edit]But N[M]ath did, see below. The officers leading the Platoons and Squads are likely to be Lieutenants or Lieutenants (Junior Grade), O-3 or O-2. Waiting on confirmation about this, I'm sure someone knows...[/Edit]
The smallest operational unit is the 8-man Squads (1 Officer, 7 Ratings). Platoons are 14 Ratings, 2 Officers. SEAL Teams consist of 8 Platoons and a HQ Unit. Naval Special Warfare Units consist of 17 officers and ratings and function as Command&Control elements. Each Naval Special Warfare Group (of which there is two) consists of 3 SEAL Teams and 4 NSWUs.
Ex-SEAL Team 6, current Naval Special Warfare Development Group, consists of ~200 operators and ~300 support personnel. All volunteers must have served two years in a SEAL Team. I doubt these are all Officers either, however.
Misc:
The UK SAS (and probably the SAS of Australia and New Zealand as well) consists of Squadrons of 10 Officers and 64 Enlisted men, each commanded by a Major. Each troop has 2 Officers and 10 Enlisted men. I'm guessing all volunteers must be Sergeants, but the limit might as well be Corporal.
The German Kommando Spezialkräfte demans all volunteers to be of Feldwebel (Staff Sergeant) rank or higher, all officers are Lieutenants. Brigade-sized (~420 combat troops). 6 companies, 4 of which 80-men Kommando companies further divided into a HQ group and 4 platoons, each platoon training for different mission profiles. I do not know the specific ranks here, but I'm guessing 2-4 officers per Platoon, the rest (12-16) being Feldwebels or higher.
The French GIGN consists ~90 operators: A command cell, a support/training detachment, a negotiation cell, and 4 mission groups. The mission groups consist of 1 NCO and 12-15 Gendarmes (1 SFC+ and 12-15 SGTs+?).