making rock formations

Hi,
I am planning on creating a few caves w/ rocks for my new tank. I read
the stuff re Rocks etc on Netmax site and went collecting....just have
a few questions though:

1) one of the rocks i found is slightly green, is there a way to test
for copper is should I just not use it?
2) site mentioned use of styrofoam on the base of the aquarium before
putting the rocks on. Can i just use a piece of styrofoam made for the
base of the aquarium (obviously a smaller piece) Do I need to clean
under or? Or do i just chuck it in there, put the rocks up and pile on
the gravel and never worry about it again??
3) If i want to stick the rocks together so they don't fall over (i
plan on stacking them to make arbour type structures) is aquarium
safe silicone suitable?
4) Some of the stuff I picked up was quartz. Is that ok If i want a
soft acidic tank? I've read stuff re quartz but can't remember if it is
inert or not.

Thanks in advance!!