It's cheaper to find uncommon pets from that family you are fusing, but it might take you a bit to find them. Maxing common pets doesn't take much though so it's probably more efficient if you max 4 commons to eventually fuse into your rare. Make sure you find a pet roll you want to use before you start maxing though. Or if there is a certain family of pets you are looking for you can start maxing your other 3 commons and wait til you find the roll you want for your 4th.