Just the gauge backlights should be OK, But I tapped those into the illumination wiring that also goes to the radio. That way the gauge backlighting dims with the rest of the dash lighting via the dimmer switch.
Your voltage readings are OK.
As far as the wiring pushed into the fuse box, it might not start a fire, it depends on which blade the wire is trapped under, the supply side, or the fused side. Either way, just thinking about that type of wiring makes that dueling banjos tune play in my head......I would do it right.