Sorry, but all that stuff about Mostali reproducing by fission has already been
Gregged. The Mostali special in Different Worlds detailed Mostali reproduction
(IIRC or maybe it was the article in Elder Secrets of Glorantha) makes it clear
that the Mostali have 2 sexes (or at least it takes two Mostali to make a new
one). There are biological processes involved but the Mostali don't like to
think about that. They suppress the memory of what's involved.
And I can't agree that the Mostali are not part of the Man Rune. But there's an
easy way to test this: if the Healing spell is halved when you cast it on a
Mostali then you're dealing with someone of a different Rune. :-) (And anyway
if they aren't Man what are they?)