secondary coils

a few questions to help ease the mind (mine)
how do i decide which size secondary to use with my system? i have both a
4" and 6"  i have 3 9kV 30mA neons as a power supply.  
i am also wondering what there really is to gain by using 1, 2 or all three
of them.  in a unit as small as mine, is it worth adding them or just going
for the best with one?  
about MMC's, either i am confused (well that is a given i guess) but when
people use these as primary caps, have i been noticing that the overall
voltage rating they chose is not the typical 50kV rating one would try to
use when doing a poly cap. but rather much less?  how come?
thanks all
kevin wahila