I don't think there's really a need to jumpstart flowering if you're running clones...

I just switched from 18/6 to 12/12 yesterday. They went straight from an 18-hour light cycle, to a 13.5-hour dark cycle (to sync up to the times I wanted the lights on/off), and then at the end of the first 12-hour light cycle I already noticed pistils on the biggest plants!!!

Kinda crazy... I never expected them this fast!