My best guess would be a leaking slave cylinder. You may have to pull the transmission to access it.
Could be a bunch of stuff, but some of the earlier Fieros had an issue with the shaft connected to clutch pedal bending which would cause the clutch to not fully disengage. When your car's not running you can shift gears all day without pushing in the clutch. Try that when your fiero's running and all you'll do is chew gears.
Clutch is not adjusted properly.
I would guess that your clutch or transmission are bad. Hope that is just the clutch.
Not 100% sure on an excel, but where is it leaking? If your car has no clutch fluid then you will not be able to change gears, I would check the seals on the master/slave cylinder, as that is one of the more common places clutches leak.
your clutch may not be fully engaging
Perhaps you are not easing off on the accelerator when using the clutch. you would get this from a worn clutch .
Something in the system is leaking.
No. Clutch is what disengages the motor from the transmission. Shift would be changing gears. You should depress the clutch to shift from one gear to another.
it has both chain and gears
A faulty slave cylinder will not cause your clutch to slip. The slave cylinder is made up of a housing and a rod. The rod pushes the throwout lever against the throwout bearing, that push the teeth on the clutch housing causing the clutch to disengage and you are allowed to shift the gears. When a slave cylinder fails it makes it difficult for you to shift gears. When it completely fails you will not be able to change gears at all for the most part. I sincerely hope this helps you. www.subarushirt.com The only exception would be if the slave cylinder managed to leak fluid onto the clutch disc.
Cracks in the clutch disk will cause the noise and a wore syncronizer in will not allow gear to mesh, more so in cold weather.
There may be some worn gears/bearings in the transmission, but more likely it's the aligning bushing in the clutch disk.