I think you meant to post this in Finished Tracks, friend!

Go listen to your favorite tracks on your phone's speakers. Compare them with your own. What's different?

-6 dB is really more of a general guideline. Like gd4 said, you wanna give the mastering engineer a lot of room to work with - they use a lot of very high end equalizers and compressors/limiters that each try to eek out every last little bit of space in the mix they can afford to take.

When you're working in your own DAW, you also have to keep in mind that up until the audio exits your mastering chain and is sent to your audio interface to play through your speakers or headphones, it's all digital information that is subject to the wonderful world of 32-bit floating point audio processing. You can have a signal go about 180-200 dB over 0 before any distortion occurs, so as long as there is something preventing it from going above 0 at the end of your master chain you could just say "screw headroom" and mix at full volume. Steve Duda has said he works with his kick at unity gain (0dB), so there's nothing wrong with it.

Just for the sake of education, I wanna point out that "sidechaining" isn't just what most of us think of as sidechaining - but ducking. We're using a sidechain trigger - where an effect like an EQ, a gate, or a compressor is activated based on the input of the signal sent to it - to cause the audio signal to "duck" out of the way on regular intervals or when receiving an external sidechain impulse.

Something like VolumeShaper/LFOTool/Kickstart can be used to reactively duck by creating an empty MIDI signal that is fed into whatever track you have the plugin on - with the first two you even get custom shaping of how the audio is cut!

There's a few good ways to do it. You can just put the audio directly onto your playlist and just cut-copy-paste the rhythm in, you can use Slice to MIDI (in Ableton) to convert the vocals into a bunch of samples and play around with the different slices, or you can just get a few phrases and play those melodically in a sampler.

If you mean clips, unfortunately not. You wanna do the tempo warping before you start mixing.

thanks i appreciate it, i think i may do that this upcoming semester.

I think it's best to go with someone who's been certified (no offense intended to Marrow, I'm sure they're a great teacher) - an instructor at school will not only be able to give you a good guideline to follow when learning, you are paying for access to their office hours where you can just walk in and go "Hey, I got a question."

It's just a sine wave, friend.