thoughts on mixing media types

While anxiously awaiting my delivery of hyrdoton, i went and bought a bag of lava rock. I picked out the smallest of the pieces to use, but fear the sharp edges would harm the roots.I have yet to use it, so have had it soaking in rainwater. Now that i have both and I'm about to set some 3wk old seedlings in pots for DWC i was thinking of mixing them up.
I was thinking of putting the HT in the bottom half and adding lavarock on top to weigh things down and add support as it grows. Would it be better to just mix half and half from the start, or best to keep the different media types seperate? Does anyone have any ideas on the benefits to this or reason not to do either?

You can mix them any way you want.
If you go with all lava rocks. Get some heavy gloves and a hammer and bust them up to the size you want. Wash them off and your good to go.