You can't Crossfire those cards, they need to be the same model. You can Crossfire cards from different OEMs, but they must be the same model underneath.
yes you can. crossfire is not as strict as SLI. you can CF both card as long as both have the same chip inside even if both card from different series or different core configuration. and there is not much difference between 270 and 270X anyway. they are the same card with 270X clock much like the original 7870. everything else is the same between the two. to be honest i don't know why AMD need to make two different class of product based on core clock alone.