Could I use orzo instead of paella Rice?
It's not a direct substitution, but you can use orzo in place of paella rice as long as you vary the liquid. When I make a paella I use 3x the liquid as the rice by volume. With orzo that may be too much - if it doesn't absorb it you'll have a watery paella. 

My approach would be to add 1.5 times liquid to the pasta by volume (not including your tomatoes) and see how it goes. If it starts to dry out and the pasta isn't done you can add small amounts of liquid until it is cooked. 

FYI, any short grain rice will do for paella, you don't have to buy rice specifically saying paella rice.