Hello, I can't comment on the Caine Road Preschool, but my son has attended a Wodland Montessori with this age group mix.
First of all, I dont think you will find there are many, if any 6 year olds in the class, as these children will most likely be at Primary school already. The oldest children in my sons class, by the end of the school year had only just turned 5.
So the mix of children wasnt so broad.
I found the younger children really looked up to and learned from the older ones. I think as my son got older though, then he preferred to be in a class with older or same aged children. He has a little sister at home, and wanted some time away with some big kids. Just his personal situation.
I did go to a couple of story times at my sons school, and the children all seemed very happy in the same class together. And I guess it is part of the Montessori way for children to learn for older kids and to take care of the younger ones.