Should 6dB be too loud?

proper gain staging is EVERYTHING.
 
 

There's no standard as such, more a general thing that if you're mixing lots of tracks it can be easier if they're all around -12 so when summed they don't all end up having to be reduced in gain/volume to avoid overloading the master bus. A singer plus guitar on the other hand might be easier to mix if tracked a bit hotter.

In any case DAWs can always reduce or increase the gain of a clip before it enters the channel at all. So long as the clip in question isn't recorded at such a low level that background noise is an issue if it's boosted enough to fit with the rest of the project simply increasing the gain might well be all that's needed. Or reducing the volume/gain of all the other tracks.

iZotope is running a mixing series of videos with 10 suggestions. Interestingly, it wasn't until step 4 that levels were even considered. At that point (the mix was hot, but sat well and clipping in 32-bit is not much concern), he rectified it by driving all faders down to give his mastering headroom back, then set the master fader to unity.

I found that interesting, since levels are not as critical as the mix itself and levels can be easily adjusted as a mix.