Can't offer much help, however I have a late 2015 15" that cam with El Capitan, and I have left it there.
Fan gets going during processing in LR, mainly HDR, Pano, or Export, but not too much for adjustment brush action. And once export or HDR or pano is done, fan spins back down.
You can try SMC fan control, it will over ride the MAC speeds I have used it for years.
Paul Caldwell