Generally, a bad slave in a hydraulic clutch system will:
Make the car difficult or impossible to put in gear while the motor is running
spew fluid all over the garage floor (you would still see evidence leaking from the bell housing)
cause grinding for no apparent reason and/or (in the case of an internal diaphragm leak) very spongy clutch pedal
The clutch shares the reservoir with the brake system.
It's not really possible for the slave cylinder in a hydraulic clutch system to cause the clutch to slip. When these kinds of systems fail they fail in the OTHER direction... The direction that doesn't permit you to disengage the pressure plate from the flywheel. In other words, you'd have no clutch.
The only exception that I can think of is if the slave cylinder got stuck somehow, in the extended position (mechanical/physical failure of the component). This isn't very likely on a late model Mustang because there's no hard pieces connecting the inner and outer portions; only a bellows.
Stuff a quality clutch in that bad boy along with a new OEM slave cylinder and call it a day. It seems like I've seen/heard a lot of stories about folks reusing the original slave then discovering a new leak.