Judy, you should protect the edges of the mirror when doing mosaics as you would when doing stained glass work. Since you are using mirror mastic, it should be OK with the back. Remember moisture will ruin the backing.
The secret to having grout colors available when they are needed is to buy the tints. You buy one of each color for a cost of about $70-80 and you can make about any shade of grout needed to offset or blend in with the glass/tile. Everyone can buy from these people if they meet the $25 minimum. The grout color can also be altered by using the tint with white/off white/ grey, etc. Many possibilities....
http://www.castlewholesalers.com/cgi-bin/sc/productsearch.cgi?storeid=*1ccd4ce49039ac7b1019465acab820b4