The main advantage of mixing your own soil is that you know exactly what is (and what is not) in it and you can tailor your feed accordingly. Also if you get issues during your grow, knowing exactly what is in the soil helps. If you can post a link to the soil your going to buy we may be able to tell you if there is anything you need to add to the feed schedule or mix in. Also dont use chemical nutes with organic soil as they dont work in the same way and your wasting your money on the soil. The biobizz nutes I posted are organic and would work with organic soil.
On your second question... that is not an easy one to answer. I personally go for one plant in the biggest low profile pot thats fits and then use LST to bend+tie the branches to create a flat even cannopy which maximises the use of light. Others I have spoken to use more plants in 1 liter pots so they get root bound and dont grow tall - this is probably a micro version of SoG. I also went for the bigger pot as it retains more water and hence less time watering it, with the really small pots it can get to watering twice a day.
In my micro grows I managed to get 120w made up of four 30w CFL's - but it did get really toasty and had to have the extract fan on fairly high... still managed just over an ounce with it though.
