Started my plants off at 18/6 growth seems slow, no deficiancies yet, maybe not enough lighting witch ill fix, but I have 150W HPS on the sprouts right now about 12" away. Can I switch to 24/7 veg light after I have started 18/6 for 2 weeks. I heard it grows much faster than 18/6, but i also heard that it just produces extra sugars and waht not for those extra hours of light.
Short answer is no... it won't hurt your plants going from 18/6 to 24/7. They will say in a vegetative state. I've tried both and can't honestly say I notice much of a difference in growth rates. I prefer to let my plants have a dark period, but that's all personal preference. As far as what happens to the plant on a cellular or molecular level, I won't even pretend to have an answer for you.
Ahhhhh... ...now pot size definitely has a lot to do with growth. Some like the 'graduated pot' system, where you move to larger pots every couple weeks until flowering. The benefit is supposedly a much more thorough root system, but the downside is that it can slow growth dramatically. Especially if you're slow to transplant. If room permits, I like to start seeds in the 3 gallon pots they'll finish in. Times like now, where I've got more in veg than I can handle (recieved 3 packs of seeds in 2 days and got a little ambitious), staggering some plants in small pots can be a lifesaver.