I've added the silver add-on to my August FTP. To answer your first question, yes, you may add this to the FTP. If you do the add-on, it must be for all members of the party; I'm not sure how that applies to two rooms. You receive two wishes per day for all members of your party. For your situation, there would be a total of 84 wishes (2 wishes x 6 people x 7 nights). The cost is per person: $67 for adults and juniors (a junior being any child over the age of nine) and $55 for children (age 3-9). You can use the wishes however you like and there are no limits as to the number of wishes to be used each day. It is not cost-effective to use the wishes for children's meals, and you should pay cash for those. I'm not sure of the number of children in your party, but for adults, the wishes break down to $33.50 per wish. It makes sense financially to use the wishes at at the more expensive sit-down restaurants (Chefs de France, Le Cellier, Brown Derby, Citricos, Narcoosees, California Grill, etcNote the latter three require two wishes per diner) or for the dinner shows, Hoop de Doo or Ohana, instead of counter service or character meal dining. They may also be used for certain spa treatments at the Saratoga Springs (formerly known as the Spa at Disney Institute) or the spa at the Grand Floridian. The SS spa offers a better variety of treatments than the GF, IMO.
The wishes can be used for up to six hours of child care at one of the wDW children's clubs (e.g., Neverland Club, Mouseketeer Club, etc) and various recreation activities (e.g., boat rental, water mice, etc).
If the search function is working, look for the thread by Janet 2k. That thread provides an excellent description of the package and the various items (including meals) for which the wishes can be used.
Good luck with your planning.