What connectors do I need for this water heater?
The manufacturer has the manual available online. It gives a lot of detail, and does mention using both copper and PEX.


  Step 7:
  1. Connect the water
  supply
  Determine the type of water pipes in
  your home. Most homes use copper
  water pipes, but some use CPVC or
  cross-linked polyethylene (PEX). Use
  fittings appropriate for the type of pipe
  in your home. Do not use iron or PVC
  pipe – they are not suitable for potable
  water.
  2.
  Connect the cold water
  supply using 3/4 inch
  National Pipe Thread “NPT”
  to the cold water inlet nipple.
  For ease of removing the water heater
  for service or replacement, connect
  the water pipes with a coupling
  called a union. We recommend using
  a dielectric-type union (available at
  your local plumbing supplier ). Dielectric
  unions can help prevent corrosion
  caused by any electric currents
  common in copper water pipes and
  can help extend the life of the water
  heater.
  3. Connect the hot water supply
  using 3/4 inch NPT to the hot
  water outlet. Follow the same
  connection guidelines as for the cold
  water supply.


Reliance Support Web Page

Reliance Water Heater Installation Guide