My DD9 is tiny as well. She skipped a year too and she is the youngest and smallest in the class. She does not like being small but she is OK with it. Sometimes, she even jokes about it. Our family talk about how God made everyone different: some taller, some shorter, some smarter, etc... and she is chosen as a smarter but smaller one... probably helps too.
My DD 12 is MG and DD9 is PG. Last year, they took EXPLORE as 6th and 4th grader respectively and they got the same score. Last month, they took the 8th grade Math test (in prep for CBE) and my DD9 got higher score than her big sister. My older DD is OK with it. She accepted long ago that her younger sister is smarter than her. My DD9 is going to be in 6th grade next year but will do Algebra for Math. (It's like in grade accleration which is easier to do in middle school) My DD 12 is in 7th now and doing Algebra right now. So, there is going to be a small gap between them. I would be real hesitant to make them in the same class. But I wouldn't rule it out totally.
I think you have to consider case by case. Rules may not fit for everyone. I wouldn't sacrifice the accleration of the younger sibling for the sake of older one (that wouldn't be fair especially the younger kid is PG and older kid is in grade). But at the same time, you have to look at the family dynamics, psyche of both kids, etc....
I am fortunate that my older DD is quite forgiving and she loves her sister so much and she wouldn't let sibling rivalry get in the way. I keep my fingers crossed that they stay that way for a long long time.
