I’d say there is nothing wrong with a compressor, or even multiple ones, for the mix - if each step is justified by its influence on the sound. I often have 1 or 2 compressors going in the mixing step, one to shape the transients, another to level out the ratio of spikes/throughs in the volume envelope, or just to make things pump a little… Also important is the order of the compressors, and EQ pre/post compressor which each will influence what the compressor does in different ways.
Not sure if I understand the loudness thing with the compressor. Ofc a compressor can boost gain, or raise loudness of a sound…but isn’t it at the end of the day about the mixer faders, in the ultimate phase of mixing, leveling sounds against each other so they blend well? So I cannot understand how a compressor would be used for this. If you have a quiet sound, okay then you can boost it with a gainer, but compressor will always also alter the dynamics and is normally only used for that purpose.
If you use compressors to raise the loudness of a track against others, then you are doing something strange imho. And you risk destroying the sound with compression if you fall into the pit and keep boosting all the instruments again and again, trying to balance them somehow in the wrong way. You can always also lower the volume, of the louder sounds, to level out stuff! And as said, EQing can greatly help with clarity of sounds mixed against each other. Don’t level to make sounds cut through against each other - level the sounds to balance them against each other, so each has the weight as intended!