Wrote this about two hours ago, then left to go to a meeting....
Each amp channel operates independently, so there's no problem with one channel operating with a different load than other(s). It's quite common for folks to have different impedances.
As for why lower impedances are more difficult, my mostly-mechanical background leaves me in a difficult place for explaining that clearly. A little web search turned up
this article , but I don't know how much it'll help you.
This article may be a bit more practical.