If there is really no way for sound to travel upstairs through the walls of your theatre then due to your budget you shouldn't double drywall the walls of your theatre.
I am not sure of all your options. If you are planning to only cover 75% of your ceiling with GG, it will be not be effective. Look at sound like a fish tank, once there is a leak in the tank all the water starts draining at that leak, the water doesn't care about the remaining areas that don't have a leak.
With that said, you don't really need Safe N' Sound insulation, normal 1ft fluffy pink stuff will work just as well, the safe n' sound stuff is just a way to get more money. It won't really add that more STC to your ceiling.
STC is obtained by adding mass and sound absorbing materials. GG absorbs the vibrations so they don't pass through the next drywall sheet and into the studs.
If you can do it, double drywall & GG the ceiling of your theatre room (especially around the HVAC area) and use regular 1ft insulation in the ceiling joists.
You need to make sure when doing double drywall that you mud the cracks of the first layer and let it dry to ensure a flat surface for GG coverage on your next layer.
goto avsforum.com and search "Green Glue" you will find a main thread with questions and answeres from GG makers.
