Do I need to buy cabosil for the fin box/leash plug installation? Or can I make due with the less congealed resin/hardener mix?

You do not need to use Cab-O-sil but the resin might drain into the foam and require an additional top-off pour... 

There is no need for Cab-o-sil in FCS fusion, FCS II or futures fin box installs. Only suggested for FCS X-2 and Factory fin plugs as well as leash plugs.