Splicing permanent outdoor lights

Edit: Can lights be removed from the extended strand to make it shorter? What I mean is that I need the extension to get to another roof line but I don’t need all of the lights in that strand. Thanks
Is it possible to cut a strand of non pro lights in two and add a 12-15 ft extension? Is there any limit to the length of the extension you can add? What’s the best gauge of wire to use? I’m nervous about doing this so I want to make sure. Thanks