If you laid the slc a few days ago then I would'nt bother but if you feel it's dusty then yes.....trowel for the tile would be 10mm and back skim the tiles, if you feel your not getting the coverage use a 12mm.
No need to prime but, make sure you have a thorough hoover to get rid of any dust prior to tiling. If your floor is nice and flat, a 10mm trowel will be fine. If there are a few discrepancies, a 12mm may be better.
It depends on the thickness of the mosaic, if it's 6mm or thinner then yes a 4mm is the trowel to use, if it's a 10mm thick travertine then a 6mm notch isn't going to cause problems with adhesive oozing through the joints.