If you want to do it all in FL Studio, Might be worth exporting each track off as wav (drums as one, bass as another, fxs/vox another etc)
Then re-import each wav in a new sequencer, and run 7 band eq, Compressor, ultramaximizer etc to beef up each wav.
What I normally do is redirect the channels of the mixer to the last mixer page, so I have a channel for drums, one for percussion, one for bass etc etc etc so they're all grouped at the end.
Then I just do careful A-B ing to get the ez and levels right.
To be honest get the mix sounding good and don't worry too much about loudness as that's what mastering houses are for. When I take stuff in to the label loudness is never mentioned, only EQ and levels and they all go to press fine.