I did my honours thesis for my Economics degree on the consolidation in the airline industry, so I'm pretty up to speed on the available data. I also remeber this discussion room as being of little help, so I'd be glad to assist you.
Fleets: Specific airline websites will provide this data in depth
Employees, revenue: Go to comany annual reports, and it'll have all current data - they're found somewhere on the company website, and many academic institutions have them in hard print.
Enplanements, other passenger data - The US Dept of Transportation has everything you need here: do a search for stuff like fares, and the Consumer Fare Report's got loads of stuff. The Acountant General's office also did a great report on the US/UAL merger, and they've got loads of figures.
Lemme know if you need anything more.