can I replace the valve cover in my 2003 maxima with used one from different year, (or from a different car)

It should be okay to swap with any other car with a VQ30DE engine (3.0 V6) or VQ35DE (3.5 V6), there are various Nissan cars with these powerplants (probably some Infinity models have the same engines). Not sure whether the valve covers can be switched between the 3.0 and 3.5 though, never tried that.

You cannot swap valve covers between the VQ30DE-K and VQ35DE. Two entirely different engines. The 3.0 has the rubber seals and grommets etc. separate from the cover itself while the 3.5's rubber seals etc are built into the cover. Buy a 2004-2006 valve cover for a 6th gen Maxima. It's way cheaper and the seals are better. It fits perfectly as the 5.5 gen and 6th gen use the same engine. Make sure to use adequate RTV sealant and Torque the bolts properly in the right sequence or it will leak.

only use rubber gaskets made for valve covers on it NOT rtv sealant.

Nissan suggests using RTV in the FSM but either way it works. Most use the RTV, some use the gasket and some use both.

I'm a tech at nissan. (5 years now) the oil pan' timing cover, etc. use silicone but they do not recommend using anything but the rubber gasket on the valve covers.