Apparently you can but I've never seen it done, you may be best to keep your PHP apps and Rails apps separate.
Especially if you're just starting out with Rails you wouldn't want to go through the setup.
Heroku and EngineYard are great cloud-based options for Rack applications if you don't want to manage your own server.
If you do want to manage your own server Linode or AWS are good cheap options with shell access, I'd suggest starting out with a free Heroku account so you can get used to the tools first.
- Are there any equally good open source CMS and forum software for RoR as there are for PHP?
Not so much, the reason you get into Rails is because you want to build your own applications, it's not about downloading a massive system and dropping it in.
So, if you want to build new apps Rails will help you do it. If you're looking to find a bunch of existing apps that you can install and configure PHP has a lot more.