In general:

When pre-washing, a normal wash with extra rinse is fine. Some people don't bother with extra rinse on pre-washes, but it does help to prevent residue build up.

Microfibre only needs one pre-wash. So does cotton. Hemp and bamboo need more (about 4 for hemp and 10 for bamboo until they reach full absorbency), BUT, you can do one or two prewashes and then use them, just bear in mind they won't be fully absorbent. Shouldn't be a problem for a newborn though. They don't need to dry before washing again. Avoid using the tumble dryer as much as possible - sun drying sanitises and removes stains, tumble drying will weaken elastic and isn't good for PUL. Inserts are fine in the dryer though.

Cold water is fine. If you find your nappies are getting stinky, a hot wash every now and then will help. My bub is 7 months old and I've never done a hot wash. I use a normal cycles without a pre-wash and with an extra rinse, but I have a little squirt so all my nappies are rinsed before drypailing.

Detergents should have no enzymes, optical brighteners, bulking agents/fillers, and some nappy manufacturers recommend no fragrances either. I use aware powder for sensitive and find that it doesn't leave residue, but gets nappies clean. Use half the recommended amount of detergent.

HTH!