Has anyone had multiple CMSes for one site? Or a CMS and Wordpress? How is your experience with that? Is there a better way? (FYI - we are currently locked into Ektron, but if we wanted to also use something else...)
In principle, there's no problem with that. You just have to watch out for potential conflicts ... but there aren't many. For example, both CMSes might want to claim the index.php file in the root folder. In that case, just put the second CMS in its own subdirectory.
Or you can just build a new site somewhere else (subdomain or local environment) and then switch them in one big hit.