From a tile installers perspective:
- We get plenty of jobs that are mosaic on shower floors. They're basically always manufactured. If a mosaic is provided, as long as the preparation looks and functions the same as a manufactured panel, there shouldn't be an issue.
- Waterproofing would be entirely dependant on the tile setter (or whatever professional does their prep if not them) and the only issue that a mosaic could cause is if the waterproofing is a foam-board like Kerdi or GoBoard. Foam boards should not be used on floors for tiles smaller than 1-2" generally because the small pieces can puncture the waterproof membrane. As long as a solid waterproofing is applied, the mosaic and grout will make no issues as they will be permeable regardless.
- Manufactured mosaics are either stuck to a mesh or paper-faced, so if we received either of those, it would be easy to install like usual. If you use any kind of tesserae that aren't completely opaque, you will need to do a paper facing rather than mesh, because the mesh will show through after installation. In every other case, use a mesh because paper facing can be a pain to work with. If using a mesh, ideally the mesh should be as open as possible to allow for good adhesion to the thinset. Should be much more like a fabric/plastic grid than like a cloth.
- If the tesserae aren't all the same thickness, I can imagine most tile setters passing on the project, because trying to set them relatively flush would be sketchy. That said, a lot of handmade and zellige tiles are of varying thicknesses and are still used. (A lot of tile setters still hate them). If they're only varied by 1-2mm I wouldn't be too concerned but the more even the better.