I went to a single-sex school, but we had a very close relationship with a boys' school around the corner. We were in each others' plays and musicals, held events together and religious thing were done together, shared resources and the senior grades had joint classes in many subjects... I think it was a great way to go.
Do any of the schools you are looking at have close working relationships sister schools, where that gender interaction can be fostered, while still maintaining the single-sex focus of the school?

It is interesting to note that my 'brother school' has since turned co-ed, while my school has remained all girls, and many of Melbourne's best private boys' schools have also gone co-ed now as well. Whether that is based on education and social outcomes alone or the primary motivator was financial, is another question I suppose!