Using a central CAS server seems like good idea for your use case. The pros are thay you will be able not only integrate logins for multiple Rails apps, but you can mix and match different solutions this way (PHP, Elixir, Java…).
I would recommend you not making a big decision to jump onto Elixir with everything you do just yet. When you have stand alone, fairly simple use case - give it a shot. See how your programmers deal with the task, let them learn the new stack. Maybe some API or a service first, and only when they are flient and confident do bigger apps.
This is actuallu advice that I would apply to any new tech stack. Try it in limited fashion before making big decisions on switching whole stack.
It seems you need a central SSO solution anyway, so implementing it may be good wether you decide to stay on Rails stack or move on.
