Is it advisable to use any kind of sealant when installing the new pipes and gaskets to the heater core?

I think you need a couple of these:
https://autoricambi.us/collections/cool ... ber-gasket

This one is from Autoricambi, but all of our trusty vendors sell them.

Already got 'em. I'm wondering if anyone uses any sealant in addition to the gaskets. Or, can it be that gaskets are enough?

I didn't use any sealant when I replaced mine two years ago.

lglade, how tight did you tighten the nuts and bolts that hold the pipes to the core? I know I don't want to overtighten but...how does one tell what is tight enough?

I don't remember how much torque I applied (and my shop manual didn't show one, either) but it wasn't much--just enough to get some compression on the gasket and stop.