Please cubase and fl user i do play beats in fl studio and rewire it to cubase for vocals but my question is do i have to master the beat i.e on the master output do i have push it harder as a mastered beat or leave a head room for the vocals in cubase before mastering everything in the mix together..?

 

I'm not the person to ask about rewire, but I'm guessing you should follow normal gain staging procedures.

That means

1.Always set recording levels so the signal is -18 dB avarage and maximum -10 db peak, what ever happens first.
If you have already recorded some beats too loud or too low, raise or lower the volume so you get the levels right in FL. Then send them to cubase.
2. Record your vocals at -18 db avarage and -10db peak.
3. Mix, making sure you are not clipping any track FX, master FX or the master fader. Many mastering houses want you to deliver at least 6db headroom. That means no peaks above -6dbFS on the master fader.
4. Master, depending on your release destination you may have diffent levels but a good avarage would be -14 LUFS with peaks at -1 dB True Peak. This should do it on Spotify, iTunes and everywhere else. If you are going to CD then you can do go a bit louder. Masters for club use could be pushed even hotter.

See this link
https://www.masteringthemix.com/blogs/learn/song-mastering-for-cd-club-play-and-streaming

Hope this helps

 

At this stage - when the instrumental is done and only need to mix the vocals - you don't rewire. You export everything to audio and mix the stems using a single daw (cubase). Then export to a stereo file and master the stereo file (cubase).