Dont know anything about your purchase but I can concur with your friend in that most ammeters found on large aircraft are not designed to be operated in line, but across a shunt.
A shunt is fundamentally a resistor of very low resistance and connected external to the ammeter in parallel with its moving coil.
Therefore what this type of ammeter is measuring is a voltage drop across a resistance.
With regard to the lighting, you may need to apply a bit more voltage - would imagine the backlighting is 28v
GR