I am trying to kill all unwanted background processes (especially the ones started by zygote) on a rooted android phone (any version and any model), so that I have the maximum possible RAM available with me? Can someone please help me on how to go about it?
That's a really silly thing to do. Android will automatically kill background processes to make more RAM available when necessary. Most of the time it leaves background processes in RAM (but not running) to allow those apps to start up quicker next time, and to use less power (because it doesn't have to keep reloading the app into RAM). See the answers to this question for more info.