I am replacing a water heater that developed a slight leak at the cold inlet. At first I thought it was the nipple, but after disassembling and reassembling the connections (used Rectorseal #5 on the threaded connections) I realized it was a leak on a weld.

So when installing the new one, do they come with the T&P already installed? If so, do I need to remove it, dope it, and screw it back in? Ditto with the nipples. And should I be worried about over tightening threaded connections?