Its not about swamping your tune with synths, its about fattening up what you have
if you listen to most modern pop it will sound like there are perhaps four or five instruments playing but the project will contain 50 or 60 tracks...we're not studios but their techniques can be used by us...I was reading about this rather nice
acoustic guitar part actually had 14 different acoustic guitars all playing the same part, an extreme example
I get the rap part..I know the sound you were going for..and it should have that spoken dry sound, not much of this type of music makes it to the US let alone onto the playlists of most folks so arrangement wise you just have to know that you know what sound you are going for and choose to ignore comments on that part
I dont understand about changing midi into audio though? you have you outboard synth as an external instrument in cubase and you can hear the audio?
after you have the sound you are looking for bounce the midi down into a separate audio track...I cant tell you exactly how to do it in cubase but its pretty easy..try youtube for tutorials
ill shut up now lol