Super Small Grow

So i have 3 fresh seedlings in a very small space. There are 26 inches vertically (which is my concern). The top 6 inches is a shelf that holds wiring and stuff and cant be taken down. I am hanging the 2 fluro lights off of it. So this leaves me with 20 inches. I think that when the pant gets bigger i will hang the lights high up next to the plant (theres reflection at the top). My pots have a 6 inch diameter and are like 6 1/2 inches tall.

How high should my soil be? (minimum in order to give my plants more room)

When should i change to flowering in order to keep the plant at like 11-13 inches?
 
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