Being fairly new to IEM's, I have not yet had any issues with over-singing or blown out voices. My singers are pro's, and sing at a very even level each night, so we've never had a problem. By using the channel compressors lightly, they hear what I hear. I had not thought about putting a limiter on the aux out's, but that is an excellent idea. I've never needed it for live monitors, so I've just mixed IEM's the same. I'll give it a try at the next gig.
Regarding pre/post DSP, I've completely changed my thinking on this, as a result of owning this board. I've learned that if I make EQ adjustments to a particular channel, that needs to be heard in the monitor as well. You can't fix individual channels with the aux 31 band EQ, so at this point I prefer the channel EQ to be heard in the IEM. I'm using the channel comp's very lightly, and my band loves the IEM's, so I must be doing something right! (I'm going to try the aux limiters at the next gig)
i dunno, it's pretty much live sound 101 that compressing vocals in monitors is Not Done.
i suppose that if this isn't a loud rock band, and if the compressing is set such that it's not even engaging half the time, then it's no big deal. still, you might try setting it "pre-DSP" to see if the singers notice a difference. the "tell" will be if they ask you to back the vocals down a little or if they seem to be singing softer or further away from the mic, this means they're now hearing themselves
better.same with EQ, if the channel EQ is pretty much set then left alone, then it's OK if it shows up in the monitors too. otherwise, tweaking it mid-song is a good way to freak out your singers. also, they shouldn't need much corrective channel EQ in the ears, that's mostly a "speaker box in the room" issue.
finally, i suspect most hardware IEM limiters are set so that when they kick in you're already pretty goddamn loud.