It'll be fine.
The sprockets will dig into the freehub body a little bit, which'll make getting the cassette off tricky, but using SLX instead of XT won't kill you.
The only thing to watch when removing the cassette (and if it's been on for a while it will be a bee-atch) is to make sure you pull the cassette off the FHB, not the FHB outta the hub, which I've seen done many-a-time.
(Tip if it won't come off: take off the smallest sprocket and lay the wheel on the end of the FHB, not the axle, on the floor (cassette-side-down). This will require supporting the FHB on a bit of wood or whatever. Push through the wheel on the cassette, down off the FHB.)
That was a long-winded way of saying what needed saying.
