Can I use flux core wire with MIG shield gas (argon/CO2)?
Maybe, but best not use the gas. The flux core has deoxidizers in it like aluminum , silicon and manganese ( not really a deoxidizer). They are expected to react with oxygen from the air and be consumed. If the oxygen is kept away by the gas some of those elements will dissolve into the molten metal . This will add alloys that may affect the properties of the deposit . Many years ago Lincoln did something like this and the resulting welds had high silicon and manganese causing them to be too strong.