right way to put engine coolant?

[userfriendly]

is it safe to mix two different coolants? or i really have to drain before putting the other new coolant

Usually safe to mix. If you are using a long life coolant, 5 years service life and you add the green (whiz, prestone....) coolant, useful life will be shortened to 2 years. No other effect that I know.

Yun nga lang, some car manufacturers specify a specific type of coolant for their vehicles. Toyota HOAT type, GM Dexcool OAT type etc..

Here is a good read:
http://www.aa1car.com/library/2004/us120426.htm

[speedyfix]

you can use any coolant naman pero if you drain the system be sure to rebleed it after or else you will have air bubbles trapped in the system.