Can previous tiles be removed easily when installing a new tile floor?
They can be removed, how easily depends on the original installation. If it was installed using the wire mesh and cement method, it will be labour intensive to remove.
Can single tile be removed without breaking the surrounding ones?
This can be done but best left to a professional. However, if you’re going to try first remove the grout surrounding the tile with a grout rake. Chip the tile to be removed with a chisel and be patient to remove small amounts at a time.