I suspect that any Jupiter size planet will not allow the formation of terrestrial planets closer to it than the distance from the inner edge of the asteroid belt to Jupiter. I would guess the distance of this influence is proportional to the mass of the gas giant. The evidence is the emprical properties of the asteroid belt , the law iof gravity, and the theoretical belief that Jupiter prevented the asteroid belt from coalescing and may have stunted Mars' growth. The simulations support what ought to be fairly obvious in my opinion.

If stunting Mars is correct, it is quite likely that an Earth sized planet in a habitable zone is unlikely in any system with a Jupiter mass planet closer than 4 AU. It is unlikely a Mars size planet would remain habitable over geologic time scales even if it started that way.

I think its possible the mass of the nebula forming a system would have some influence since a denser nebula would seem to allow the formation of bigger planets with the same spacing or more planets of the same size in a given area. However, I think the primary impact of nebula mass would be in the size of the star formed. Since there is good reason to believe a star near sol's size is needed to allow intelligence to evolve nebular mass may not be an important additional consideration.

Has anyone here worked with planetary formation models? If so can you provide any insight into the primary causal variables and the primary effects?

Regards,
Mark