Has the cedar been waterlogged for a long time? If not it will leach toxins and damage fauna in your tank. I don't think limestone is a good idea if you're planning to plant this either. If you're not planting... could be ok.
clothing storage is typically made of cedar for its ability to keep insects and other creatures away due to its toxins. i've also read to stay away from lime stone because of its calcium carbonate leaching into the tank. be weary.
Haha I saw the stone and instantly thought "this person must be in Texas."
The limestone will be fine if you use amazonia aquasoil - it lowers the ph pretty good, and Seiryu tends to raise ph about as much as texas limestone anyway. I made a layout similar to yours utilizing cedar and limestone/holey rock to make the effect. Just a tiny pet peeve of mine - it's not an iwagumi unless it's 100% rock.
The wood has been soaking in a black tub in the good ole Texas sun for months. I change the water once a week. It started brown. It is now clear.
With the limestone, I was concerned about the ph. But I put this rock in my non planted tank for 6 months, and the ph never changed. I thought this strange, but decided I could risk it in my planted tank because I really like the look. I will look into the Amazonia.