That all sounds right. Technically if may be possible to set up staging and production for some types of apps on a single droplet by using multiple databases and virtualhosts going with two seperate ones would be best practice to ensure that things stay separated.

I don’t think you want a floating IP for this. I think you want domain.com pointing to your production droplet and qa.domain.com pointing to your staging droplet.

Otherwise, when you repoint domain.com to your staging droplet, all users would be using your untested staging code (same effect as deploying the update directly to your production droplet).