@Myles, I see nothing wrong with using fully composted leaves. It will add water-retention to your media, so you need to compensate for this with additional coarse perlite or something like (clean) cypress mulch. You want a well-draining, good aerated media. Keep in mind that peat is acidic. If you are growing acid-loving plants, so much the better. Otherwise, add a couple of cups of lime for every wheelbarrow full of media. If you happen to be using milled sphagnum rather than peat, the sphagnum is neutral, so no lime will be necessary.
