Electrical and plumbing often require separate permits. If you are installing anything in or under the foundation you will need to get these permits before you start anything. So plan ahead if you intend to install either of these.
Electrical
The only pre planning that electricity requires is to have a ground installed prior to the concrete being poured. Code often requries this be done and certified during the foundation inspection. Even if you don’t plan on having electricity right away, it’s a good idea to have the ground installed and certified for the future.
Plumbing
The only plumbing that needs to go into the slab is a toilet or shower drain. The water supply and a sink drain can go through the wall at any time.
Insulation
2×6 walls will hold more insulation than 2×4’s so this is a good reason to use them. You can install insulation at any time after the shed is built.
Drywall
If you intend to drywall your garage then you will want to make the inside wall height so that you don’t need to cut each piece to fit. This is where I recommend using precut studs as they will give you 1 1/8 inches to work with.
Save your framing scraps to use as drywall backing where necessary.