I have a few problems. I need suggestion
 

One, my engine coolant light comes on. What coolant is good for our RX8?
Second, when I rev the engine, a weird soft/mono popping sounds happens when it decelerates. What could be the problem?
Third, I just cleaned my MAF sensor and the car still has a weak idle once in a while?

Need way more info about your car such as year, mileage, mods, etc. to answer your second and third question.

Having said that, concerning your first point. Does this coolant light only come on or did it first start by only coming on when you were really getting on the car hard and it would go off after a little while? Reason I ask is that if you do a search, you will find that the coolant sensor(which is molded into the overflow tank) is a known failure point. Do you have enough coolant in there now....meaning are you to the full line on the bottle? If you are, then it's the sensor. Your choices are really to replace the bottle or live with the light and check the level often. As just one example............. https://www.rx8club.com/series-i-trouble-shooting-95/coolant-bottle-greatest-secrets-revealed-182793/

2nd point could be related to your third point and would need more info, but in my experience a little popping might occassionally be heard.

How long has it been since the coils, plugs, and wires were changed?

If the coolant level is at the full mark or just above after sitting overnight and you are still getting the coolant light, then your coolant sensor is bad and you need to replace the overflow tank. I little pop or two is nothing to worry about. If you have 20k or more miles on your ignition coils and spark plugs i would replace them. Aslo look into some aftermarket spark plug wires. www.raceroots.com sells a nice set. I like to use some BG44K fuel system cleaner every 10k-15k miles. Also for your engines health do a search about premix

^............yeah but remember that in most cases I have heard of(including my own), the coolant light first starts acting up when approaching redline in let's say third gear and then when you shift it goes away after about 10-15 seconds. This continues for awhile until it finally stay on for good(as you said). I never let mine get that far, I just got a new tank as it was pissing me off!

As far as the coolant type, well do a search on that. There are several that will work and there are several different thoughts on the ratio of coolant to distilled water.