well how old are they!? their height is worthless unless we know how long they've been growing.....

but my preliminary guess is that the height is totally fine. 4inches above them is fine...CFLs dont get that hot.

I can't really help with your 24 to 18/6 question....I'm not a fan of growing at 24 and I'm certainly not a fan of switching the time unless its from 18/6 directly to 12/12