Personally, I wouldn't do any kind of indoor/existing light work with people in motion with a micro 4/3 camera.
That's your prerogative.
You'll need ISO speeds in the 3200 range and up
Not if you use flash.
and I'm sorry, an M43 sensor just can't handle that
I disagree. 3200 is fine. 6400 is pushing the limits. That's my assessment based on real-world print sizes and deposited checks from clients.
although I'm sure their will be an angry mob with pitch forks out to prove me wrong. I'm not saying it's impossible, it's just not smart.
That's a rather dismissive attitude, and insulting to photographers who've made the "not smart" choice.
I've shot many receptions with fast lenses, handheld and crop sensor is the smallest I would consider for those types of shots.
My wedding photos are typically printed no larger than an album page. At this size, ISO 3200 with smart processing is no problem. My clients are delighted with my results.
http://www.jacquescornell.photography/dilnoza_and_abdugaffor
http://www.jacquescornell.photography/jabbar_and_nigina
My bread & butter work is covering corporate events, and I do this with MFT, too.
http://www.happening.photos/p246381502
http://www.happening.photos/p375848970
http://www.happening.photos/p300757967
http://www.jacquescornell.photography/p325237376
Lots of other successful event & wedding pros are shooting with MFT these days, too.
Check out my gear list to see what I shot with before switching to MFT.