Is this a seperate expansion or are we to use the original slave with new cards

You should probably have asked in the main X-wing forum. That being said,

Is this a seperate expansion or are we to use the original slave with new cards

There's no new model for the Firespray, but you should use the scum dial, Pilot tokens, and Pilot cards found in the Most Wanted expansion. The Slave I title is usable by either the Empire or Scum, as is the Andrasta title.

Yeah most wanted is needed for the cards and dial but you have to buy the slave one expansion as well, the whole point of putting cards in most wanted was to avoid the need for two firespray kits.