Id replace the foil with mylar, as foil can cause hotspots on the plants, plus mylar reflects 98% of the light. It costs around £2.50 for a sheet that measures 1m x 1.4m. Oh, and add the CFL's as sidelighting, especially in the budding stage. This will make the buds that bit denser, increase potency and also speed up budding a little depending on how many CFL's you have and what wattage they are.
It all depends on the space really, but with a 400w HPS and a few CFL's added in for good measure, around 4-5 plants should be no problem.

P.S I dont consider myself an expert, Im at the 'intermediate' stage so I may not be correct on the number of plants. I would think that 4 would probably be best, as I dont know how tall those plants grow.
If theyre indica dominant and grow quite short, you could do the Sea/Screen of Green method (SOG/SCROG) and fit 6 in.