So frustrating. My neighbors have twins (they are fraternal although they looked identical until they were about 8!).
Having them in separate classrooms is good - my neighbors did that, as did another family I know whose sons are my own son's age. The kids have separate social circles as well, as yours do already.
Your children are totally separate individuals despite sharing a birthday. I think any change you make now is much easier than anything you try to deal with 3 or 4 years from now. I understand why you are worried about bullies, and it's a shame when kids use "dumb/smart" labels. However, there is bullying on a whole host of topics and for all kinds of reasons (kids who don't wear designer sneakers, kids who need braces, kids who are introspective, kids on the autism spectrum, you name it). There was just a report on our local news about a Massachusetts high school having to deal with a resurgence in anti-Semitism with kids being targeted because they are Jewish. So it's rampant.
That said, I'd say that it's hard to protect kids from cruelty from other kids. So we all have to start early to teach our kids compassion and how to have a backbone. I think you can't make an educational/social decision based on what might happen - in fact, Abby's just as likely to have problems because she's already not up to the rigors of large classes, workload, social interaction and whatever else. So I'd make the decision that's best for her right now and down the road.
The girls will always be close as sisters, but they've already forged separate paths socially, which is great. A lot of twins feel pressure the other way - they have to be together all the time, dress alike, face assumptions by others that they are clones in every way.
I didn't think my son was ready for kindergarten at 5 - he was a 3-hour-napper (and he would have had afternoon kindergarten), he was small for his age, and he wasn't ready in terms of concentration and social skills. He was outgoing and made friends easily, but he wasn't ready to be in a group of 22 kids. My brother, on the other hand, was the youngest in his class (where we lived, the cut off was 12/31, and he is a late October birthday and was SO young). He STILL suffers from never having felt adequate, and frankly, he's developed some pretty terrible habits (he brags, fabricate things, tries so hard to show people up). I wish every day of my life that he had been given an extra year or had been able to repeat kindergarten.
Good luck with whatever you decide.