klasiksb,
Thalassoma newtoni from West Africa can grow up to 8". This information can be verified on page 611 of the book titled World Atlas of Marine Fish by Rudie H. Kuiter and Helmut Debelius.
I would not recommend mixing wrasse of the genus Thalassoma with Halichoeres melanurus . Thalassoma wrasse normally do best in larger aquariums mixed with larger more territorial fish.
Regards,
Kevin Kohen
Director of LiveAquaria
Drs. Foster and Smith