First of all, you're not going to be able to host both php and ruby code on the same app in heroku. It sounds like the rails site is no longer going to be used in production and is only there for backup purposes. In that case, you'll be fine running that app on heroku's free tier.
So what you should do it this.
- Keep your rails/heroku app exactly where it is.
- Create a subdomain with your dns provider to point to the old rails site (something like oldsite.mydomain.com).
- Point the new domain to heroku.
- Downgrade your heroku account to the free plan.
Then you should go ahead and setup your wordpress site wherever. I've never used heroku for a wordpress app. For all of my professional sites I use wpengine. I wouldn't even consider going anywhere else because my experience there has been so great! Change your primary domain to point to that wordpress site.
Done!