As I alluded on the current grade-skipping thread, we had a fourth-grade-age student in a multi-age class with 6/7/8th graders, doing pre-algebra, which worked out well, granted, in a tiny private school, so not entirely analogous. The following year (fifth-grade age, seventh-grade placement) was algebra I, in a two-person class with an eighth grader. I don't think it's so much the age of the classmates in general, as it is the temperament and maturity of the specific student, the attitude of the receiving teacher, and the composition of the receiving classroom. In a medium-to-large middle school, there is also the factor of negotiating the campus.
More importantly, though, I think it is unnecessary to worry so many steps down the line. It is highly likely that the plan will have to be revised again between now and fourth grade. I would make placement and programming decisions based on what your child needs in the next six months to a year, and then re-visit it on a regular basis.
