Hi!
Personally, I agree with Sir Finkus about embark everywhere but at your own risk being the ideal solution - but that would require world gen sites to be much, much more detailed and functional before it becomes interesting.
First of all, there is the question of armed resistance against your occupying forces. I mean, if your 7 dwarves march into the middle of a 60 head elven village and start cutting trees, it should become a very short fortress mode session indeed - especially if that village has been fending off invaders in the past who had been much more numerous and experienced.
Secondly, the exploiting races need to be more effective about their exploitation. For instance, if you decide to start digging under a dwarven settlement, you should find that all the gold, platinum, iron ore, and aluminimum has already been dug up by the dwarves (as time permits, of course), leaving only small clusters for you to exploit. If there is a fishable site, you can be sure that they have set up fishing there as well which may interfere with your own ambitions. Also note that dwarves and humans destroy forest tiles during world generation (well, actually just the trees), so the supply of wood should be limited when you embark within their realms with their lumber jacks competing with yours. And nearly all good surface farmland around human settlements should be claimed by the human farms except for those oddly shaped strings or small edges here and there.
So, even without getting at odds with the locals, edging out a living could become rather problematic. But these are all things that are not implemented yet, so that embark everywhere would currently be no risk and high gain for the player.
Deathworks