I'm assuming here that you've got floorboards under the ply? If you've got concrete then you don't need the ply. Personally, I wouldn't use any other adhesive than Bal when tiling onto wood - the peace of mind is worth the extra few quid it'd cost you.
...and make sure it's flexible and grey (unless you like grubby looking grout).Softus said:Get purpose-made powder-based grout.