Veterinary partner is a very reliable website that should be able to answer your questions on FIP
Your cat wont get FIP by being with an FIP + cat, it can however contract coronavirus. And yes as you mentioned, not all coronavirus cats will develop FIP.
From my "Common Diseases of Companion Animals" textbook:
80% to 90% of cats in catteries have antibodies to FECV and these cats shed virus intermittently.
That being said by continuously disinfecting and only having one/two cat(s) in your home you can eventually clear out FECV (most common household disinfectants work).
You can check your current cat what his current anti-body levels are for coronavirus if you're concerned about a new cat coming in as well.
An excerpt from veterinary partner on FECV:
An active infection lasts several weeks to a few months. Virus is shed in the infected cat’s stool during this period. If the cat is reinfected, virus sheds again for weeks to months. During this time, the cat may or may not seems at all ill. Some infected cats do not shed virus.
The mutation to a form of virus that can cause FIP is more likely to occur in an immune-compromised cat. Most cats with FIP are less than one year old (their immaturity is their immune compromise). Crowding is also an important source of immune-compromise. Another important factor in mutation is multiple exposure/infections. When cats keep getting reinfected, the virus they carry has more chance of mutating to a form that causes FIP.
I've only seen 3 cases of FIP in almost 4 years in practice, it's not very common.