No real disadvantage to 18/6, One advantage is 42 hours less electric use per week!
Not sure about the light leak question? Did you mean that light is leaking out of your veg cab? This is not a huge problem, what would be a problem is if light is leaking into your flower cab, I would check this out Asap so that you can sort it out if you need to, before you need to flower.
Considering what you paid for the setup there should not be any light leaks at all
Although there were slight gaps in the chambers I sealed them all up with silicon glue & foil tape, theres just a gap where the first chamber door slides close (thats where the plant is). I guess Ill just stick some tape on it during the night so it wont distract my sleep.
If I was to build my own cab then I would stick some alluminium tape on the seems and this would work wonders, can you get hold of some?