Not seeing any info here on this, and as I'm looking to get started with running this game I want to know if I can use the Snes9x core in retroarch, or if I have to use the standalone Snes9x application. Thanks!

Using the Snes9x core in RetroArch is allowed, just be sure to check that the core is running on an accepted version for the leaderboards. (afaik it should be on 1.6x, and 1.5x and above is fine).

Sorry for the slow response. The discord is much more active and has a #help channel for any future questions, happy running!