So, how much would Bt affect other flower gardens that are about , about 100' feet away, by killing the good bugs?
BT isn't a problem if you are careful and don't broadcast it. Even 10 feet away would be fine. I've been using it on my tomatoes and peppers but still see butterfiles in the flowers in the yard. I add it to the watering can when I'm doing a drench of the plants (using a mix of Garrett Juice, fish fertilizer that helps keep spider mites away, and BT). Either sprinkle this over the plants with a watering can (it gets the soil also this way--BT affects eating caterpillars, but if they're starting from the ground under your plants maybe they won't have a chance to take more than a few baby bites) or foliar feed topcially.
If you add BT when you do foliar feeding then use a gentle spray (I use a little 1 1/2 quart sprayer with a hand pump knob on top, and stay very close to the plants) then only hit the plants you want to treat.