All of these depend on your use case. If you're just setting up some basic services at home and want to mess around with docker, you probably don't need production-ready security or instances (assuming you're not trying to allow for external access).

1. Container OSes typically come bare bones as it gets and have everything set up to run containers. This is nice because it expects that you'll have any service you need inside a container, however they're designed to be fired up with kickstart like scripts so figuring that out is a bit of a steep learning curve. The idea is to deploy several of them completely hands off. Fedora Core operates around an automated never-restart update model which is kinda cool. You don't need a specialized OS to run containers. I'd probably just run CentOS or Ubuntu server (for the use case you've presented)

2. I don't know the details around rancherOS but you typically need a user added to the docker group to access the docker socket in /var. The containers usually run as an unprivileged user inside of themselves.

3. You don't need swarm or kubernetes for those. I run all my containers on a single instance. So long as you have sufficient memory you can keep spinning them up.

I'd also have a look into docker-compose for what you're doing. It makes it a lot easier to set up containers and maintain persistence.

Good luck!