Hi Steve,
Many packs use same voices, luckily YEM recognize duplicate files(waves) and load in your installation file only once.
If you have 2 packs, each 100 MB, and you make a PPI file, if 50 Mb are duplicate files, the size of PPI will be around 150 Mb not 200 MB logical.
Packs are made to work alone or together, if you erase from one pack what you consider to be duplicate you don't win space, just, if you save the pack like PPF, that missing voice will be not there if you install this new PPF alone.
So my advice let the pack on the original format.
The voices sound different because on different packs was saved with different parameters, the waves are the same, and the styles choose after LSB (pack position on YEM), I thinck.
Regards
Mihai