Hi B.,
I have worked as a preschool/kindergarten assistant for a few years (until I got married and had a child myself) and I am watching the church I go to start up a preschool program. My mother is a preschool teacher along with being the director for the before/after school program at school where she teaches (her school is associated with a church).
Every child learns at a different pace and has a different attention span. Maybe the kids joining the class are ready for the older preschool learning pace. I see preschool as getting the child ready for the school structure (we are going to do this now ex:learn, snack, learn, play and so on). That way it is an easier transition into kindergarten where they are expected to follow the school structure without too much prodding/asking the kids to do something. The school I was a teacher aide for had the 3/4 pre-k and 4/5 pre-k split in the morning and in the afternoon together (if they stayed all day). They mainly had snack and played in the afternoon, because it was treated more as a day care environment because they did the school structure in the morning.
My mom has only old 4/5 year olds, and I know that a few young 3/4 have tried to talk there way into the class but my mom does not want to mix the ages. Even if the child seems ready you say yes to one parent with a young 3/4 you have to say us to others. It is different when in the afternoon, after they went to school in the morning, then the time is more like a day care (playing and such).
A few preschools are putting together 3/4 and 4/5 because of small class sizes and they can't afford to have them split. I have seen a lot of 3 year olds not ready for as much as a young 5 year old, and the teach has to pay more attention to the younger kids. I would say that if your child is still doing well and learning on the combined days then it is fine. If it seems that not much is getting done/learned on the combined days you might want to speak up (especially if your child mentioned that the structure of the class has changed, ex; we don't read any more, the teacher spends all their time with the new classmate(s), we play all day). If the structure of the class is not suppose to change then there should be no change to your child's learning ability, but sometimes adding your children who are not ready for that structure can cause the "learning" level to go down. That is when you want to speak up... but if the younger kids are ready for it then things will go on unchanged (just more in the class). Give it a shot!
Regarding about starting in the middle of the alphabet... that should not be a problem. What I gather is that they learn what starts with that letter. So really it is more about the words and not the order of the alphabet.