Don't just trust the breaker to be a fire protection device. It is the wiring that comes off the breaker and extension cords being used. Some people are stupid enough to think that if they keep tripping a 20 Amp breaker that switching it with a 30 Amp one will solve their problem. Sure, it will probably not trip as they were overloading the 20 amp breaker but are under 30 amps. The problem is the circuit wiring using a gauge for 20 amps or less. Now those wires will start to heat up, WILL melt the wire insulation, and will short,spark, and cause a fire. In this case probably in your walls. (think of the heater wires inside a toaster)
Have a professional electrician run new circuits and wiring for you if your not qualified to do so on your own.
Also cheap power strips, timers, and extension cords as another danger. You can have a up to spec properly rated circuit and breaker for lets say 30 amps. If you use a power strip, timer, extension cord rated less that the breaker, or even outlets rated less that the circuit then they can AND WILL heat up, melt and you got a fire.
Also, bad or missing ground can cause fires but most likely will give you a good shock or could kill you on the right conditions.
I do not have any of those automatic fire extinguishers, but I like them and will be investing in one or two.
NEVER EVER take shortcuts with electricity. If in doubt, have an electrician do the work for you. Indoor grows mean your risking your property and life if something goes wrong.
IMHO, the perfect indoor room will have three separate circuits and breakers. One for the lights, one for the fans, and one for the normal room if it was not being used for a grow...all coming out of its own sub breaker panel off the main breaker. That way if there is a problem, you can kill everything by hitting the main sub panel breaker.
Also, especially if doing hydro. All electricity above your waist and all water below your waist. Keep them as separated as possible.
I also suggest outlets being ground fault protected types that will turn off the power if it detects an unsafe electric leak. They can be sensitive as hell if your setup is not 100% correct electric wise, but they will safe your life.
DO NOT take electricity lightly. Not knowing what your doing or trying to cheat the electric meter or bypassing anything is like a game of Russian roulette. You may get away with it for months and even years, but the odds WILL catch up with you.