The schluter edge has spaces to install with the mortar. You don't want a lot of extra mortar poking through the edges, just a quarter inch or so to make sure it's solidly attached to the concrete floor.
For the mortar, I used FlexBond, which allows for the Slate to expand during drops/raises in temperatures. This was a little more expensive but well worth it at $30 bag versus $20. I used two bags