[whswhs]

The basic approach seems sound. In Template Toolkit 2, I worked out that Elastic Skin was fairly close to Morph (Cosmetic, -50%; Retains Shape, -20%). Elastic Skin requires a Disguise roll, and Morph doesn't; this is roughly analogous to adding Flawed, -10% (not actually legal to combine with Cosmetic), for a net -80%, which fits the base costs of Morph (100 points) and Elastic Skin (20 points).

I suppose you could say that not having the right vocal sound, and needing a Mimicry roll to fake the voice, could fall under Flawed as well.

But I'm not sure about your example. Can you change SM?

On one hand, if you change SM, and mass is conserved, then the ogre has turned into a halfling-sized being that weighs as much as a normal ogre. That's a factor of 2 reduction in height, a factor of 8 reduction in volume, and thus a factor of 8 increase in density, making the "halfling" as dense as iron. No organic material is that dense.

On the other hand, if you decrease the ogre's mass by a factor of 8 (not legal with Cosmetic, as you point out), its ST and weight are mismatched. The problems with that show up best if you get bigger. Per Growth, if you go from SM 0 to SM +1, you need a basic ST of 15 just to move your body around; acquiring that extra ST does change your racial template.

I think it's probably best to say that you can't change your SM, unless you also change your build to compensate (becoming a Skinny ogre or a Very Fat halfling, for example). If you don't change your build, what the ogre does is turn into a 7'6" halfling, one with multiple levels of Gigantism.