Ah, that's my mistake. Apparently, the ID for Felin is plural, while the public name isn't, and I appear to have mixed them up. I just updated it, so resub to the mod and try again.

I have Felins installed, and yet when I install this mod, I get a quick CTD upon startup. The log complains "[22:43:04.952] [Error] Application: exception thrown, shutting down: (StarException) Asset source 'ManyTabsSWFelinCostumesAddon' is missing dependency 'Felin'"

So for whatever reason, this mod isn't recognizing that I have the Felin mod installed.