Connecting a Volt Ammeter to the input and output of a power supply
Diodes are cheap using them will protect your switch.
short-circuiting the ammeter will not damage anything

If you can live with a brief interruption when switching he relays aren't needed:







simulate this circuit &ndash; Schematic created using CircuitLab

"A in" is ammeter sink    "ground" on your circuit. 
"A out" is ammeter source "ammeter" on your cricuit.
"volts" is "voltmeter" on your circuit.

This only works because the buck converter has input and output grounds connected internally.