So when its worth 7500 put liability on it?
That's just my rule of thumb - in your case you don't lose your ability to earn a living and you already pay less "per car" because of your collection, so your analysis may end up in another place. At least know how much you're paying for what before you drop coverage.
So whats the differance between comp and full coverage?
I'm using "comp" as a shorthand for comprehensive - or "full coverage". This is really just to differentiate it between liability only (usually at the statutory minimums) and collision coverage.
Some people just buy collision and liability. The finance company requires full coverage (comp) because it protects them first, and from theft, collision, etc. If you're not as worried about theft than collision, it can save you some money - again, in your case, you're buying insurance "in bulk" so the decision should be based on how much you're paying versus how much you're protecting.
The insurance companies have gotten pretty good about pricing things correctly and once they have your business, they generally cut you better and better deals on covering additional vehicles (at least until you start filing claims, then they turn on you quickly, which is somewhat understandable). Again, if you don't like/value the Fiesta enough to cover it (at least collision, if not comp) against your own fault or the fault of someone you are reluctant to sue (you loan the car to somebody and they wreck it), then it is the candidate to sell.
However, you say you drive it the most. You may have hoarding syndrome with regards to cars. If it is not negatively impacting your life, maybe you shouldn't worry about it.