You can go HID right away if you like. Just keep them 2 feet away or so at first. Make sure a fan blows air over them. Or you could go 2 weeks CFL and change to GHPS after that. Either way introduce HID light slowly.
Roots should show from 3-10 days I would think.
Rooting gel is good. It speeds things up.
Try
www.insidesun.com or
http://www.htgsupply.com/ for pH stuff. Sorry those are not UK sites.
Feminized may or may not be worth it. Some say feminized seeds go hermie. I disagree if the genetics are good. They do cost a bit more but may be worth it because all 10 seeds are about 99% female. But it is just as easy to sex, and then keep a cloning mother forever in veg to get unlimited clones off of her. All you need is one female for that.
You can grow tomatoes with the weed but, just make sure you follow the one plant per pot rule; no mixing tomatoes with weed or multiple weeds in one pot.
Your soil mixture sounds fine; just go easy on worm castings if seedlings will be planted in it. Seedlings need no food [castings, guano etc.] for at least 3 weeks of life.