I don't think it matters whether you use one kind of wood or not. It'll decay and once broken down, does it matter? Maybe different fungi prefer different woods, I don't really know. I wouldn't think twice to use all one kind of wood in a hugelkultur.
As for the water trenches, for us, the water penetrates into the ground pretty quick. I've never seen the water stand for more than 24 hours, especially since we have really sandy soil that drains quickly. The depth I've always subscribed to is that it should be as deep as it is wide. So a 2 feet wide swale would be 2 feet deep. Our little swales uphill from our hugelbeds are only about 6"x6".
