Hey, welcome to the community! Hope we can help you get comfortable with Foreman
We do indeed add a bunch of things to the initial database by default, as most people want to be up-and-running as quick as possible (this cuts down on those “paper cuts” that tend to make people quit before they’ve even got it working ). All the things you’ve mentioned are part of this initial seeding, but it’s actually done directly in Rails rather than via the API (and also consider checking out Hammer, our CLI tool).
There’s no way to disable the initial seeding of data, as far as I know (some of it is necessary for Foreman to function at all, like creating the initial admin login), but once the install is done you’re welcome to modify/delete the entries, or create entirely new ones if you wish. This can be done via the GUI or the API, whichever suits you better.
Ultimately, it’s your instance, after all - if you’re (say) only going to be a Debian shop then deleting all the RHEL/Centos stuff makes perfect sense. We just provide some defaults to get people going, and obviously we can’t predict which of the many OSes we support they’ll want to work with.
If you need any more guidance on specifics of those steps, or want to go into a little more detail on your goals, feel free to shout out.