How to approach mixing this?

Ok I have been asked to do a demo for a group, well 3 female singers who do events/gigs etc to karaoke backing tracks.
 
How would I go about bussing and finalising the mix?
 
Im guessing buss the music track (which is allready mastered/commercial level etc) to the master untouched and then bus all the voxs to a vox bus etc but at some stage theyre going to end up on the master buss together.
Can I throw a compressor and limiter on the master bus even though the music would allready be at commercial levels?
 
Hope this question makes sense?
 
Advice, thoughts and experience welcomed :-)

yes definitey as each vocal track requires and then again on the vox buss to make them gel a bit more. (if required)
So say if I have delays and reverbs busses etc should I make a sub master and send everything to that and then put my limiter on that (do I even need a limiter?) and then send everything complete with all processes to the master track (which will have nothing on it in the effects bin etc) and just send the music track to the master bus (unprocessed)

Level the vocals to taste and export etc

Should be easy but im just trying to get my head around it lol

How would I go about busing and finalising the mix? Im guessing buss the music track (which is already mastered/commercial level it

If its already mastered, then there's not much you can really do to it. Mastering is the Final Step, not the 2nd to last step.
You can tell them to send to you a mix of the song, before it was mastered (pre-mastered mix), then you can finalize it properly (master it properly)
Cj