I'd say the beauty is you're open to silo yourself off as much or as little as you want. As long as the actual blue sky team doesn't go too crazy with their own moderation I think it creates a healthy community

I don't believe so, it looks like they use their own API. I would guess because they implement a subset of the subsonic API and it might be hard to juggle both. But there's no harm in trying and seeing what happens.

Yeah, I'm pretty sure they interpret the endpoints differently so unless a mastodon post is directly targeted at a community or something, lemmy just drops it. That's why I still run a dedicated mastodon and lemmy instance. Kbin promises best of both worlds but doesn't seem mature enough for my tastes.

Generally if you search for the full URL or handle of a profile or community your server should be able to find it. For example to find a user like mark ruffalo I just searched for @MarkRuffalo@mastodon.social on my lemmy instance. After a few tries it did find him but not his posts. I've only reliably gotten this searching to work through the main lemmy ui so your mileage may vary depending on the app you use. Misskey should work similarly I believe but I don't follow or know of anyone on there so not sure

It's a risk reward question then. That would 100% slowdown the initial federation if it needed to pull in every community and even if that was accepted, should every instance constantly poll any instance it knows about for new communities? Also you aren't guaranteed to need all those instances anyway and then that's just a waste of space and processing power. Correct, it limits interactions but only to what's necessary which allows instances to be ran on lower powered hardware, allowing more people to join in. With the possibility of third party tools I don't see much of an upside of building that into lemmy.

this number is only completely relevant for someone on an instance all by themselves or with no communities at all. And discounts instances that are or will de-federate either partially or fully. It also assumes some need to be a part of all 12,000 communities. I think tools like you linked solve this issue anyway. I personally believe to a certain extent every community being federated to every instance kind of defeats the purpose of federalization.