I asked the question to c what other ppl would do, to c what other ppl think.
Out of those, I'd dump the Fiesta - the money it is "saving" you on gasoline, is probably costing you that in insurance. It also has the greatest potential for depreciation. I would alternate driving your older cars to work.
The only caveat it is - IF you drive a long way to work, and really, really need an efficient 4-banger, then sell the Cobra or the lightning. You're not going to get anything for the F-100 and that could turn into a child's school vehicle. The Cobra is going to kill you when try to insure teenage drivers anyway.
I think 5 is the limit, and that's assuming you've got a teenager coming up on a driver's license test in a year or two. If you want to cut to 4 - figure out which vehicle you drive the least OR which one you like the least.
If you're loaded and don't want to sell anything - don't worry about what everybody else says. I know a crazy lawyer that owns about 20 cars.