J.
I'm sure some people with experience teaching preschool will post, but I was wondering about what you expect to find in Kindergarten (same school, different school?) I know that my son's (public) K class had (entering) kids who didn't know all their letters and kids who were reading simple books and everyone seemed to be OK in the end.
I was an early reader myself and my son was not - right in the middle average - and he is now a very strong reader (in his school's gifted program) at age 8. Early reading is nice, but reading-prep is just one part of the preschool experience.
I do think it's valid to wonder what the teachers plan to do with the mixed age groups and abilities, and I think you should ask for a meeting to discuss that. I know people who have been very happy with mixed-age classes, and other parents who felt their kids didn't get what they needed. I think a good teacher who is trained to work with mixed-age groups can make it a great experience, but you don't want your kid to be the one working alone in the corner because she's the only one who has already mastered all of the work.
Also, ask about the size of the class if they are combining two rooms. Just my take on it, but I think a low student-to-teacher ratio plus a mixed group might work out great, with lots of individualized help. In a bigger group with fewer teachers, maybe not.