Blooming B@Q 🙁
Make sure Tony that the customer is aware that because the slate has not got a machined back that:
A. It make take a little longer to fix,
B. They will more than likely be a bit more twisted than normal,
C. That you will have to use more adhesive to overcome the difference in thickness.
Just in case you were thinking of buying, I wouldn't recommend you by a 'pourable thick bed stone adhesive', due to the fact that it is a pourable adhesive means you won't be able to build up the adhesive under the tiles as it will sink.
If you do feel slightly worried about the job use a standard set adhesive (24 hour) as this will give you more time to use the adhesive without it setting in the bucket.
As for grout space, lay a few out to show the customer, i'd for for minimum of 4mm grout joint, but you can show them a wider joint. It will be a bit more of a trap for dirt etc, but it will make the fixing easier in case the tiles are not uniform across their width, and it will make the tiles appear slightly flatter!
Lay, clean if any adhesive residue, seal grout, seal, maybe add a finishing wax.
Enjoy!