I would agree that it depends significantly on the specific child, and her emotional/developmental level. When our already-one-year-grade-skipped DC was advanced from third to fifth in a(n unannounced) whole grade acceleration, the mixed grade receiving class actually was instructed in the sixth grade ELA, social studies, (and math, for our child) curricula (confusing, yes)--an effective 3y cumulative acceleration, which means that our 8 year-old did have to manage middle school themes.
This worked mainly because of 1) the temperament and maturity of the child, and 2) the sensitivity and flexibility of the teachers; and 3) it didn't hurt that the school was very small, cozy, and warmly-relational, which cut down on the early adolescent drama quite a bit. I'll note that this was not our sensitive child, but our hypersocial ebulliently-optimistic child.
I think it would be an excellent idea to sit down with your DD and give her some space to express how she feels about her current schedule, how her academic, social, and emotional needs are being met, what might improve her situation, etc.
Balancing aspects of development is such a challenge...I feel for you.
