It can take several months to get rid of a large infestation. Infestations do not get large quickly and, unfortunately, they do not go away quickly, either. Yes, Gentrol can be used in the very same cracks and crevices where you spray Phantom and can be sprayed behind and under the refrigerator to control German Roaches. For roach control, you should repeat applications every 4 months. These insecticides will just kill the roaches that come in contact with it, but as long as you are keeping the Vendetta Plus seperate from the Phantom Aerosol and the Gentrol IGR they will not interfere with each other.