First off, for your first attempt this is looking great! i wish my first was along these lines.

Just my 2 cents:

The easiest way to tell when a plant is sexually mature is when it starts showing alternating nodes. Meaning the branches won't be symmetrical. Once that happens, the plant is ready to flower/show it's sex.

It was suggested that you go 12/12 to see the sex then re-veg them with 18/6 or 24/0. That is not the preferred method. Messing with the light schedule will confuse the shit out of your plants and, if they don't hermie on you, they will at least take a few extra weeks to sort themselves out.

The best way to sex a plant is to clone for sex. Wait until your plants are sexually mature (alternating nodes) then take a few clones off of each plant. Take very good notes as to which clone came from which plant. Once those clones root, throw them into flower (12/12) when those clones start showing sex...then you know what the parent plant is and there is no need to re-veg or change the light cycle. easy as pie.

You mentioned 250-400w HPS...that's gonna be a little small for 8 plants. I have 2 400w and i only do 2-4 plants under them. Of course i like big plants...i could probably fit 8 smaller plants...but i like the low-maintenance and High yield of a big tree.

Ok. That's just my perspective, but hopefully it gives you something to think about.

good luck. looking good!
Skeet.