You should be worried about size.
The style of growing along with wattage of light will determine how short or tall you allow your plants to get.
A 400w only has around 24" of usable light penetration measured from your bulb down. Meaning you don't really want your plants any taller than 2ft because your light is more than likely 10" away from the tops of your plants which means you have 14" of plant material to work with.
8 plants in 15litre pots under 1 400w is cutting it too close.
I hope for your sanity that you get 4 males. Otherwise it will be TETRIS time in a few weeks as you will need to rotate them all.
Stretch during the switch to 12/12 can be limited by doing a few things
1.you can keep using an MH lamp for 2weeks
2.keep using your veg nutes for these 2weeks
3.keep your light as close as possible
4.keep daytime and nighttime temperatures as close as possible.
There is no easy "buy this bottle" during flowering to keep everything chugging along, you need to feed what they want when they want it. PK13/14 is good to use during flowering but dont over do it or you'll burn them.
Depending upon whether you are full organic or not, some Potassium Silicat is usefull during flowering, it's also called LIQUID SILICON. It helps strengthen cell walls and is a good pH up alternative.
If you are organic then some friendly bacteria inoculations would also be beneficial. Mycorrhizae and Trichoderma are good.
Other than this I've checked your blog and plants look ok. Well done.
J