Whats the CA code on home electrical?

Get a book on basic house wiring and read and understand it. Every wire connection must be made in an approved electrical box. The size will depend on the number of conductors and devices in the box and the size of the wire. 2 x 12/2 WG cables need 5 times the 2.5 cubic inches of space required for each #12 wire. If you add an outlet then you need another 5 cubic inches. The electrical book should explain the sizes and how the wires are counted in a box. The way you have asked this question shows you need the education the book will give you. As an alternative to the book you can hire a local qualified professional electrician to do the work. Do get a permit and have the work inspected.

A metal box needs he romex secured with the use of a romex connector. the size of the box depends on how many wires are being spliced. then stapled with in 12 inches.

GFCI is not a inline fuse.It is a safety device used to protect against electrical shock around water (kitchen ,bath rooms, laundry

rooms) unfinished areas (basements ,attics, crawl space) garage and outside

Just by reading your question any electrician can see that you aren't very familiar with doing electrical work. Any book you read is not going to make you an electrician overnight. Go ahead and hire an electrician to do the work. Proper work will not only increase the value of your home but hiring a qualified electrician will also allow you and your family to sleep better.