I think that they're heavy support. The issue being that Heavy Support's quite bunched up as it is. (ack, or maybe elite according to that
quoted page)
It does however mean that you can take a few of them and use the fast attack slots for the purpose of fielding loads of regular spawn.
Also, both giant, spined and regular spawn are really easy to convert. The big ones can be made out of the new Warriors of Chaos monsters if you want, or just a big lump of putty with tentacles and faces stuck on. I made ten spawn yesterday just out of odds and ends I had chucking about. =P