Reply to Key (music theory) - Doubts

Hey guys,

Ok i need some things to be cleared up regarding the 'Key' of a songs.

How do i decide in what key i want my song to be in?? Once decided, does everything(basses,melody,pads) have to be in that scale which the key is in? or can i just keep the melody in one key and the chords in another and mix match everything? When i say the key of d# all the keys from the d# scale will be used right?

Also do u suggest learning all the scales or all the major minor chords at the back of my hand?

Asking this ^ becoz im never going to perform live. I produce progressive/electro. I already know all the major minor chords as well as the major scales so when ill be producing i can always look these up and then record according to that so why do need to even like learn it too well?

Please help me. Thanks