1994 saturn sl2 overheats & clutch starts slipping?
 

car overheats & clutch pedal squeaks & clutch starts slipping. Car has trouble going because gears are not fully engaged, its like clutch is still patially pushed in.
When cool clutch works fine, no slippage nor squealing. Checked clutch reservoir, it is full. Checked antifreeze & it is very close to full but not flushed in 10yrs.
Oil was down 1 quart. I am most concered with clutch, why is it working fine when cool & not when engine overheats??

Sounds to me like the clutch is nearing the end of it's life. It gets thinner as it wears out and yours is thin enough that it still has some "grip" to it when it's cold but the heat makes it harder for the friction plate to keep full contact. I don't KNOW this, as I'm NOT a mechanic. I'm just speculating from working on my own cars for about 50 years now. Andy, derf or Billy may chime in and tell you something completely different. If so, listen to what they say.

Can I just replace the Friction Plate?

You can, but from the age of everything involved, you're asking for trouble by not replacing the other worn components.

Are you the original owner?

As for the car overheating, if you let it idle in your driveway does it overheat by itself? Does The fan behind the radiator turn on when the temperature gauge gets a little bit above 3/4 on the dash? What does the temperature read on the dash when it is overheating? Any white smoke out the tailpipe?

Don't expect your coolant to behave as coolant when it is chemically broken down. It cannot transfer the heat from your engine properly. If it can't transfer the heat fast enough or efficiently enough, the engine just keeps getting hotter.

The only thing it probably has in common with your clutch issues is that when it starts to slip, you are significantly raising the RPMs of the engine to keep it moving at the same speed. This creates increased heat inside the engine and, with pretty much non-existent coolant, will drive the temperature of the engine up like nobody's business