>> Does the fediverse make the server you join irrelevant?
In theory, but in the beginning, an instance aligned with your interests will make it easier to find like-minded people to follow. The instances may also have different rules how you should behave. And you can run into problems when your instance admin bans another instance with which you would like to communicate, or you instance gets banned by them. Just the perils of online drama in small communities I guess. Running your own instance could help here, though.
>> Has SDF got an instance? https://mastodon.sdf.org/