I would vote for a relatively small amount in the very bottom layer, boiled first. You wouldn't be able to measure this small an amount in inches - perhaps an eighth of an inch at most. This will theoretically provide some additional CO2 as it further decomposes and also act as another/differing caption exchange site, to compliment your other items.
Some of my better successes in aquatic potted plant mixes have been a mix of peat and Fluorite with the peat at the bottom.
Can't really hurt as long as one doesn't get too carried away with the peat ratio. Less is more in this instance, I believe.