Since: Apr 08, 2004
It should connect up fine. I've not used one, so I can't comment on the converters. Being Behringer, I wouldn't think they're top of the line, but they should do the job.
Also, you will only get left and right going into your Mac, so you can't edit the signals later, just the stereo mix.
I think alesis has some firewire mixers, that keep the signals separate, as does the phonic line. I've read a few iffy installs with both, but I think there's quite a few people using them happily.
Re-reading your query, if you're going to use the usb to get two channels into your Mac, why don't you just take the mains out from the mixer and plug that into the line in on your Mac? Isn't there a stereo line in on Macs? I would think so.
You might need a cable converter, but it should work just fine.
Welcome to HRC, btw.