Having researched this subject, I think I can help ya a bit, there are many more experienced guys on here so correct me if I'm wrong, but Im pretty sure this is right.
1) your looking at about 75 watts per plant, i'm using about 50 and they seem to be doin ok,
2) 100 or more
3) Most people say you can never have too much light, I;m getting a 600 hps for my flowering chamber which will be 3x3 max
4) Obviously the smaller the plant the less light you can use, you need more and stronger light to penetrate through the canopy, when the plant gets bigger, you can get by this with CFL's by putting some halfway down the plant
5) I think your current setup looks ok, but its hard to see what the room is like and the spacing. Do you have proper ventalation?
6) Halogens are really bright but they are the wrong spectrum, which is probably what your problem is, I would take it out and get yourself some more CFL, you'll want ones that are rated around 5000k to 6500k then for flowering you'll want ones that are around 2700k to 3000k, Hally's are in the middle spectrum that you don't want!
Here is a great guide for using CFL's
https://www.rollitup.org/cfl-growing/107992-tutorial-growing-cfls.html?pagenumber=
7) your plants are probably slow at growing cause your using the wrong lights
also...... looks like your plants are stretching, I would move the lights down a bit.
Hope this helps!!