Buy the coolant that is orange or red in color. I would suggest Prestone that is sold by EON or go for toyota coolant or Perodua (Denso) ones. They are still red.
About green coolant, they are obsolete. Because the chemicals inside react too well to rust and dislodges the rust inside your radiator. Also notice green coolant would form white deposits. So in general green was last centuries product. Red or orange is the lastest one.
About white coolant, i've only seen once in an Alfa. Thats it. I have no idea about it.
Coolant is NOT acidic!! Rust is acidic. In fact coolant is Alkaline in nature. General coolant compound is Ethyl Glycol + some other special formula by manufacturers.
Using distilled water is the best. If u use it with good coolant even better.
General cars have 4 litre radiator capacity. More or less. And manufacturers usually suggest 50/50 mix of coolant and water.
General flush should be done every year. That is when most water would be poluted or turned acidic which forms rust.
True radiator flush means taking the radiator out to be chemical washed and flushed with pressure test for leaks by radiator shop. Not those flush you buy off the rack.
Also don't overlook the radiator cap. Change it every 2 years. Why? For optimum water pressure. You can't detect small leaks because hot water evaporates too fast.
Lastly do some research in the web for coolant type, its function and availability here. For overseas they use the word ANTI-FREEZE not coolant. So search wisely.
And the answer ot main topic,
No it is not advisable to mix coolant brands. Just add water if it is running low.