I would like to know what deductions can an employer take from your final paycheck.
I received my final paycheck there were deductions for medical insurance, short time and long term disability, etc. all the deductions that normally come out, only problem was these deductions were taken out of a 20 hour check instead of a 80 hour check. So of course I received nothing. I thought they would only take out for federal, state, social security.

As you have learned, all normal deductions for any benefits you receive are deducted from all checks, including your final check. It doesn't matter how many hours you are being paid for.

Yes, all the same deductions will come out.