can this be used in an outside fixture - is it waterproof and does it screw into an existing light fixture and then I put my bulb in the other end

yes, it is just like installing a light bulb

screwed the adapter into the outdoor fixture above my garage door then screwed the flood light bulbs into the adapter ' has rained 3x since i have done this and it is still working

As for outside fixture, exposed directly to the weather, no. It is for outside use but not openly exposed to the elements. It screws into existing fixtures and you install your bulb. Follow directions. Adapters are included to help with exposing the sensor to light. The downfall is if the fixture screw into your outlet and is in the wrong direction for sunlight. I had to change the direction of my original fixture to use any dusk-to-dawn adapter.

yes to all the questions