Jeepr - cassettes normally use separate spacers (aluminum or plastic) for most cogs, but machine the spacer into the steel of the smallest one or two. That's why I needed to find an 8-speed source for my 12T. So all the spacers I used were indeed 8-speed spacers.

Michael - whether it's phase as you say or Hyperglide shaping as I'd surmised, there certainly seems to be something which makes amateur attempts to assemble cassettes dicier than Sheldon suggests. This is pretty unequivocal: "Building Custom Cassettes: Shimano wants you to use one of its standard combinations... but you don't have to if you don't want to! It is not difficult to customize Shimano cassettes. If you substitute an un-approved cog, Uniglide or Hyperglide, it will still work, but the shift to/from that cog will probably not be as smooth as a Hyperglide shift normally is. Since people managed without Hyperglide for several decades, this shouldn't scare you off." ( http://sheldonbrown.com/k7.html) Also puzzling, if phase is so critical, that Uniglide cassettes - where there were 18 different ways to mount any cog, 9 rotational positions and then another 9 after you flipped the whole cog around ("sprockets were reversible, so that if you wore out one side, you could flip them over and the other side was just like new!") - worked. One more quote, to verify that Hyperglide cogs, according to Sheldon, will work despite the removal of phase constraints: "you can fit Hyperglide sprockets onto a Uniglide body by grinding or filing off the one wide spline." Thus restoring 9-position freedom.

Looking carefully at Sheldon's words, he does say "Shimano cassettes". So I see two possibilities: either brand purity is critical, due perhaps to brand-wide phase conventions (and Sheldon meant this, but doesn't make it very clear), or Sheldon is wrong when he says - categorically - "it will work." I don't have a position, or enough experience to even have a hunch. (I did end up with a working cassette with cogs from two source cassettes - and it's all-Shimano.) I would find either answer surprising, interesting - and useful to know.