You sure hear a LOT of questions.... questions needing a dissertation of response maybe... I'll try to hit them briefly:

Aside from Delta Force, Green Berets, and SEALS, what special forces units exist? Does the Air Force have any?

Yep, their guys are listed and describe on the BHD article of the Philly Enquirer site [see the above article]

Are there some rumored to exist but are so "special" that they aren't commonly known?

Sure ... always somebody. (Didn't you see True Lies with Arnold???)

Aside from the SEALS, do the other special forces units have very different functions?

Yes, each has a website even, and their mission etc. Generally: Green Berets does the political mission, trains Foreign troops and insurgents, Delta does the high tech Hits, and Commando raids, and Rangers provide the combat Power back up to Delta or do their own heavier hitting.

I saw Black Hawk Down this past weekend -- it was a great movie. I saw two inherent problems in the plan. There was no demonstrating force sent ahead of the main thrust to split the enemy's attention or divert it completely. Second, they had absolutely no reserve units waiting to handle problems. Even though the general didn't have the right equipment for the job, these two problems were within his control I would think.

HA! You should come play in my BHD event at Cold Wars! 9pm on Friday (F-89)

Incidentally, even though the job was botched, did they actually capture the military leaders they were after in the first place?

Sure they got some of them, but not Aidid... if that's who they wanted. They may have indeed been "set up" by Aidid with his "reported presence" intended to trigger a US Operation, so RPG Ambushes could hit the Choppers. And they did hit them. Many of them, only two crashed in "clan turf," the rest got to safety. But at the end of the day, only 2 US Blackhawks were flying in the entire Theater. At the end of the night, the war was all but over for the US Special Forces in Somalia. It could have been different. It should have been. The "buck should have stopped" at the Whitehouse. I suspect History will not be so kind toward Bill Clinton as American voters were in the aftermath of Somalia 1993. I know I won't be.