will answer some of your questions.
1. a switchable ballast will run either hps or mh bulbs all you have to do is install wich bulb you wich to use, some conventional ballast will also have a switch to select for mh or hps and this goes for most wattages with the exception of the 600 watt light, wich ive not been able to find a standard mh bulb available for the 600, there is plenty of mh conversion bulbs available for the 600 though, but wouldnt neccassarly buy a digital ballast that will operate both when standard bulbs are not readily availble.
2. dont know
3. gh 3 part is very popular and you can do a search if you want more info on it, and you can have great results, without using any of the other bloom additives, but you can try them if you wish, but in most cases it doesnt help and can even cause problems.
and a 600 would cover 12 sq ft