I already started building this circuit for over-voltage protection and I was thinking I would like to add reverse polarity protection as well. My project will use up to 8 A at 5 V. I was wondering whether it would be possible to put a P-channel MOSFET into this circuit or would it interfere with the operation of the existing components? Thanks!

 

That's not the best circuit.

The trigger threshold is going to vary all over the place, the trigger voltage of the triac gate is temperature-dependent.
Considering that the 5V is divided down to the trigger voltage, this amplifies the temperature effects- making it even worse.
The MOSFET gate voltage is going to be only about 3.6 volts due to the GREEN Led circuit - this is going to make the RDS-ON higher than you want.

Google "over-voltage crowbar" circuits- this is how it's normally done.

 

To answer your question, there should be no problem with adding a P-MOSFET at the input for reverse polarity protection.