Mix ARP and Stock Main Bolts?

The bolts matching dose not matter, but the clamp load does. Check your main bearing clearances. If you swapped to ARP with a different torque (clamp load) the bearing bores may be distorted enough to cause bearing issues. You will have varrying bearing clearances unless you had the block line honed with the fasteners you are planning to used torqued to how you are going to assemble the engine.