crushed coral is a excellent choice to help buffer the water, stabilize PH.
add CC little by little, like 1/2 a lb at a time, til the PH range is acceptable.
it can, but i would not depend on it to do that. rather, just have it take care of the buffering part, because you will need to replace it when its spent.
remember, to have the CC in a mesh bag inside the canister.
I use Biomax in my Emperor baskets. The pieces fit in the slots pretty nicely and I just slip a piece of filter floss on the input side to keep them from getting too dirty. Crushed coral would help buffer your ph, but you could mix that in your substrate and boost your bio capacity with the Biomax. Most folks in know tend to slightly overstock their mbunas to keep down the aggression so more bio would be a good idea, if you choose that route. The more Bio the better
I run 2 Marineland Penguin 350's (kinda similar to the Emp. 400) on top of a Rena XP3 and a Fluval 405 on my 120 gal. The Penguins were a recent purchase and they only came w 2 floss pads although they can hold 4 like the Emp. 400. I thought purchasing 2 more pads was a bit of a waist. I had also spent a lot of money on fish needs and xmas gifts so I was stingy and didn't want to by Fluval/Eheims/Seachems bio. Instead I went to the local dollar store and bought a bunch of the plastic scrubby pads. I have read and seen consistently here on MFK that they work great and of course are cheap. I was able to cram in 3 on each of the two sides of each Penguin. I might be able to do more but haven't checked yet.
I also checked the HOB's on my 40 gallon and 10 gallon feeder. Was able to cram down 3 into the 40 gallon HOB and 1 in the 10 gallon HOB. They might not be needed, but it doesn't hurt to have them in. Plus, and the dirt cheap price (3 for 1$) i didn't mind lol.