Re: Is the 6th ionisation energy of Sulpher lower than its 5th and why?

In general, two electrons in a valence shell with paired electron spins are a
more stable (lower energy) arrangement than a single unpaired electron.
[Check out the topic of electron spin in Quantum Theory chapters of General
Chemistry, Physical Chemistry, or Physics texts.]  So this phenomenon would 
explain what you have learned about the 3rd and 4th ionization energies of
sulfur.  

To answer the question about the 6th ionization energy of sulfur, I
would direct you to some general chemistry texts or a book on the
chemistry of sulfur (try your local university's chemistry library).  Your
assumption about sulfur's 6th ionization energy is not necessarily correct.

To answer your final question, generally the ionization energy of a one
valence electron atom will be less than the same atom with two valence
electrons when the two valence electrons are in the same orbital (shell)
with paired spins.  There are other details about this that can be checked
out in chemistry texts.