You can put four plants under that 400hps.
A 400 covers a 3ft by 3ft area you can grow 4 two foot plants under that.
Give them a quarter turn every day and your golden.
And get that one plant from under that 6500k T5 all thats gonna happen is its going to stretch and give you a poor yield.
Optimal veg time is dependant on various factors.
Are you growing short indicas or tall Sativas.
With an indica Ill let it grow to around 14 inches it will grow another 3[SUP]rd[/SUP] in flowering taking it up to around 21 inches.
A sativa can easily double and more in flower, so be careful, 12 inches in veg and then flip it to 12/12 lighting.
You dont want to exceed 2ft since light penetration becomes an issues after that.
I grow under a 400.
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I get a bit ahead of myself sometimes, lol.