Realistically, people are curious, there are always deviants, and as thinking beings they
can decide what their environment is*. Dwarf Fortress has, and will generate, a lot of different races, and although ethics and beliefs always start around physical requirements, they need not stay there; especially as technology and ethics(process patterns) are discovered that loosen those restraints. Although DF civilizations are fairly absolute and exacting in their race membership now (with only slavery and kidnapping able to bridge the magical gap), this is not a natural or realistic state, especially if there is more to be gained from the diffusion than is lost (i.e. with any races that fit in such nice niches). I can see kobolds taking a long time, with the additional language barriers and current closeness to feral behavior, but it would only take one brilliant diplomat to get it started.
If you're talking about distance when you say different environments, I can understand if there's no population diffusion over 1500km of wildlands. But roads (especially the magic roads in MoM that are effectively teleporters) and settlements along the way will change that.
I seem to have ignored the likelyhood and if-then qualifiers, but the average direction is the same either way, even if not the magnitude.
*(Acronym.) Can be slightly reminiscent of the ~Aztec style, with great stone fortresses among dense jungle.