I have always just drilled a 1/2 - 3/4 inch hole through the firewall next to where the clutch cable goes through, and bought a rubber grommet to keep it professional looking and not have wires run over bare metal. As far as Teflon tape you refer to, get it close to the end of the threads, just make sure it doesn't extend past.
There's a spot behind the A/C filter-dryer where I usually snake stuff through - a few wires and a vacuum line to the boost gauge. Have to poke through the carpet padding. It exits high up in the passenger foot-well. I'm not a fan of drilling into the firewall, especially near a stress-point like the clutch cable.
I'm definitely not a fan of mechanical gauges mounted inside the cabin. Hopefully you're just running electrical?
did everything stop working or just the oil pressure gauge? there's a fuse that provides power to the cluster you might try checking that. To answer your other question, when i ran lines for my oil pressure gauge i drilled a small hole in the fire wall right behind the valve cover and routed the line along side the intake and down to the sender. I used a mechanical gauge and this seemed to be the cleanest way to run the line. worked out pretty well too. there arent many places to run lines through the firewall that are existing unless you want to run tons of line.