Okay, I'm filipino so I could correct a few errors.
One thing you got wrong is that it goes the other way around. Kumusta ka is the informal way while Kumusta
po is the formal way. Also, Kumusta
po can also be written as Kumusta ka
po. There won't be any difference in the basic meaning of the phrase. The word 'na' can also be added before the 'ka',
like "kumusta ka na", or "kumusta ka na po".