Using oak as your only source of yeast/bacteria is another great option, but it provides a really low cell count. Its great for inoculating a primary-fermented beer with brett, but it wont give you the amount of yeast (sacch or brett) needed to ferment a batch of sweet wort.
With mixed ferments, you can wash slurry to remove trub/dead yeast, or you can just pour the next batch right on top of it. I've done both. It really just depends on how much trub you carry over and how many times you'll reuse the yeast.
You ARE kind of throwing caution to the wind with mixed fermentation, leaving your beer in the hands of the brett and bacteria. I think its fun, and if you brew quality wort, odds are good that it will develop into something special.