I would go 24/0 instead of 18/6 but thats just my opinion. Cold at night is ok. But with seedlings I've found that it can slow/stunt growth if its too cold. However I've had temps routinely go in the mid/low 60's without any noticeable problems.
Here are some seedlings I've got going. 4 Sweet Deep Grapefruit Fems and 2 L.A. Confidential Regs. I've got them under 1 125 CFL that I've had for a couple years and is very close to death. You can notice that a few have stretched pretty bad, and I believe its because of low lumens being put out by my old CFL.