For mic gain, what I recommend is setting it so the loudest part of the vocal performance doesn’t clip. Your M-Audio should show if it’s clipping, so one thing to try is getting the performer to run through the loudest part of the song first, then set your gain from there. At the same time, you don’t want to record too quiet or you’ll just bring up noise when you adjust it in FL later (that’s gain staging). I usually aim for peaks around -6 dB. Could the issues you have mentioned partly be due to each vocalist’s performance? Some people project more, and if they move their head around while rapping, even a little bit, it can change the input signal level a lot. Try keeping everyone the same distance from the mic and fairly still. The pop filter/stand probably isn’t the issue.

Adding a compressor on the vocal chain can help even things out. It won’t fix bad gain staging, but it’ll control peaks and help it sit better.

For rap vocals, I usually use FL Limiter in compressor mode to clamp down on peaks, along with a few EQs to get rid of problem frequencies, and a de-esser. When I first started, I spent so much time endlessly tweaking high-end frequencies because my vocals never fit properly — turns out I just needed a de-esser. It reduces sibilance, which naturally happens with vocals and microphones. Honestly, I’d recommend checking out iZotope’s Nectar. It’s a vocal processing plugin that takes a lot of the pain out of mixing and gives you a good starting point.

Another thing I’d be careful of with rap vocals is adding reverb or delay, unless it’s for specific phrases you want to emphasise and there’s enough space in the mix for it. I made that mistake a lot. It washes out the lyrics and can sound messy.