
The Proton has been used in 2, 3, and 4 stage versions.  The two stage version
was used for the first 3 launches, while the 3 and 4 stage versions are used
today.  The four stage version is used mostly for escape (and geosynchronous?)
orbits, while the 3 stage version is used for low earth orbits.  Since this is
the version that launched Mir and the Salyuts (and the add-on modules for Mir),
as long as Centaur is smaller than Mir (which I believe it is), it should fit
under the shroud.

I vaguely recall that the Russians are developing a LH2/LOX upper stage for the
Proton.