My policy is the crate all dogs whenever the house is empty (about 8-10 hours a week), so I prefer to board dogs that already have a history with crates-- it's just too much work to do otherwise. Our house is pretty big and with three younger brothers, I feel more comfortable leaving my boarders in a place that I know is doggy-safe. Plus, this is our home and I'd prefer it not get destroyed.
We have a long hallway that I use occasionally for dogs that aren't as comfortable in the crate. With tile flooring and baby gates at both ends, it makes for a fairly doggy proof area. I've also used it for 100+lb dogs that were too big for our XL crate.
I'm not as experienced with double booking (only done it a handful of times), but I never leave the dogs alone together--unless it's for a second or two while I run upstairs or out to the garage. Too much can happen, and I don't want to be the one to tell an owner their dog went to the vet for $$$$ after a dog fight. If I can't supervise them, they're physically separated (i.e. crates, baby gates, another room, etc); that includes night time.