From what I've read, you want a KH of at least 4 in order to provide enough buffering to prevent PH swings. Depending on the substrate you use, it might provide the buffering required, which is why a lot of shrimp keepers use Aquasoil and don't bother adding anything to raise KH levels in RODI water. I believe Aquasoil buffers to around 6.5. Eco-Complete will buffer to 7.4-7.6, but that only lasts a few months depending on how often you do water changes and how hard your water is before it goes in the tank.
As to the coral, I don't remember off-hand what PH it will buffer to. I want to say 7.4, but could be wrong. Adding more coral won't change anything. If the water's PH drops below the set amount, it causes the coral to dissolve into the water, which causes the water's PH to rise. Once the PH reaches the "buffer level", it stops dissolving. Hope that makes sense, I'm trying to simplify the explanation, but I'm operating on limited sleep and not sure I'm doing too good a job of it.