They won't limit the crowds. The Steve Miller Band concert will happen in the Bone Yard area next to Jimmy Neutron. Many people will have already stayed in the park the entire day, they'll be tired and leave. So the crowd size won't be an issue. Yes, it sounds like thousands of people would show up for the concert, and I'm sure there will be many watching, but not a massive crowd like one would think.
If you show up around 6pm, you can go on The Mummy, MIB, watch the parade, then get ready for the concert. Use single rider lines if the regular line is long.
If you want to be near the front of the stage you'd have to be there very early. There seems to be a group of people (probably locals) who get to every concert early. The sound system is really huge and they blast it, so you don't have to be anywhere close to hear it. One time we waited for some performer to start, they came out 20 minutes late, and the system was real loud and we left. So you don't have to be any where near the bone yard when it starts. In other words, don't rush over there because it's really not a big deal.
I'll also be there on the 31st. I plan on shooting video of the 360 show if they have it, then walking over to see some of the concert.