The tacho is not that hard with the Brp 174732 plug to my motor but is it possible to install my fuel gauge on the same plug so it will only get power after the ignition?
It is common to connect the tach through the control box of the engine. It is typical to connect the fuel gauge through the main dash wire harness of the boat itself.
Limiting what electrical devices come off the control box is a good idea.
If you get too much draw, it can blow the fuse on the engine, possibly stranding you out on the water.