I should mention that if one uses a mechanical oil pressure gauge, use a metal line instead of the cheap plastic one that's usually supplied. Or use a braided stainless line, but that stuff's abrasive to whatever it might rub against.
I like Moparx's suggestion of an isolator, though I don't know how well it'd work at oil pressure #s.
I haven't had one gauge leak in 30 years. Only one that did was the plastic crap I used one time before that; it was pinched at the gauge fitting.
As for feeding the water temp sensor through the firewall, I've always found a hole already there somewhere so I didn't have to break out the drill.