Mounting WLAN router, switch, 2 Raspberry Pis and a power strip
1/2" plywood would be fine.  If the hardware has keyhole mounting holes, I'd get 1/2" pan head screws for that.  Get whatever size would fit the keyhole.  Disclaimer: YMMV, IMHO there's no fire risk, despite all the hardware, there's very little power actually being drawn, so I wouldn't worry about that.

Command (brand) hooks are usually the go-to for attaching things to walls without damaging the surface, but you are probably above the limit for what they can hold.  A small picture hook would probably do, although would leave a small hole.