I had both the Brice Z6 fretted and fretless basses and I used LaBella Deep Talkin' flatwounds on both. The fit is fine, as would be the nylon tapewounds Phil mentioned above.
The LaBella flats are high tension strings, where the tapewounds have a bit looser feel (not that they are excessively loose, just not as tight as the flats). You might also want to check into the TI flatwounds, they also fit the Z6 nicely ....
