FidoNET (got I loved FidoNET) had a central coordinating authority, you had to get on a nodelist.

With Mastodon it gets incrementally harder to just add someone. Are they on the same Mastodon instance as me? Does my Mastodon instance federate the Mastodon instance they're on? Is this handled client side with multiple identities? Now how do I handle these multiple identities, so I broadcast "toots" (cringe) to all of them?

I'm also confused about your description. I host my own single user mastodon instance and have no problems to follow anyone, just put their username into the searchbar and click follow.

This way I can follow anything on the fediverse, not only Mastodon accounts but PeerTube, Friendica, PixelFed, FunkWhale, etc. all from my Mastodon account.

If you don't own the instance can you follow things on instances you're not federated with?

Yes you can but that's not how hardcore mastodon users describe things. Did you ever wonder what the local feed is about?

I don't get it.. So far I've been able to add anybody on Mastodon. I'm on indieweb.social, but most of the people I'm following are elsewhere. Seems to work fine. Yes, it isn't spoonfeeding me contacts or content like Twitter wants to, but that's also a good thing. I see a lot less bullsh*t.

Also almost nobody I've come across actually says "toots" -- they just say "posts" :-)

The equivalent of the FidoNET nodelist would basically be whatever the most popular Mastodon servers are choosing to federate with. It seems in practice to be more of a consensus driven thing, but has the same net effect. We'll see if it scales.