They don't use cluster numbers (semis/mega do, and a hybrid hunter-night creature would probably get placed both in singles and in the cluster when it hits mega/semi generator).
No idea on the doubling up caste part... I don't see any reason immediately why it wouldn't create a creature that can clone its own caste, and hopefully the current marriage/gender checks would prevent a fatal cascade of kidnapping or birth. You might be able to hack in a vampire that way. It will pick among all CONVERTED_SPOUSE castes, so presumably you could have converted spouse on the main vampire and also have a vampire slave zombie caste that doesn't have hunter. This would lead to half of the converts being vampires and half being slaves that can't make converts. However, it would all still work through the kidnap/marriage/conversion/baby system of course, making it less than ideal -- the slave would still have a real vampire baby with the original vampire, and the vampires would also have a baby before parting. When they have babies, they always use the caste of the parent taking the turn -- for a converter/converter marriage, that's probably the first to get a turn.
This is all conjecture based on looking over what I've got.