MH conversion bulb question

[Budimus Prime]

I'm planning to buy an air cooled reflector for my 400 watt HPS and considering getting a Metal Halide conversion bulb as well. The thing is the ballast gets real hot and I'm afraid to leave it on when I'm not home. I already have a bunch of fluorescents totaling about 20,000 lumens. Can I keep the fluorescents on all the time and use the metal halide for only 12 hours a day? Would that mess with the plants at all?

 

[W ï l l]

12 hours of MH is fine enough...

Leaving the the fluorescents on 24/7....not so fine.

 

[Budimus Prime]

It's for vegging though. I was kinda thinking about going to a 18/6 schedule and have all the lights on for 18 hours. They have been under fluorescents for 2 weeks so far 24/7.