two approaches to try.

first one, go ahead and fit the cloth normally.  then whip out magnets to push the errant polys closer to the figure.  then, unless the fit is humanly perfect, which really ain't gonna happen because that crotch space is very prone to vertices intersecting, you should just constrain these vertices for future sims.  also beware the poses you will use here...many times you have to adjust walk cycles, etc, because these verts brush each other, or just get so close they pinch the cloth apart

 

the second is, when you skinny the character down, slim the buttocks further than the body, or separately, so that they are thinner than the hip.  then constrain the same vertices, that clevage.  you might want to do this in two passes to get a better fit...one that gets close to the character's size, but not quite, and still very skinny buttocks, then another with the constrained verts, since constraining verts can act a little flaky in the fit to sims