I personally agree with ZeroEffect here that the Slot Calibur tracks can always be added as DLC. SC6 should have an OST of it's own and not just filled with remixes of older music tracks.
I want SC6 too, but we can't assume they're ignoring SC6 in favor of these side projects. For all we know they've composed music for it already along with the Pachislot ones and just haven't released them yet.