You can still decide to transfer your account later on, so starting on mastodon.social isn't too bad. https://guide.toot.as/guide/move-mastodon-server/#1-guide-configuration Even if i would suggest to change it, as it kind of beat the decentralization aspect to it, i believe ~80℅ users are just on this default instance. The official website has it's own list of suggested servers per interests and some rules here : https://joinmastodon.org/servers You can search via a minimum maximum character limit here : https://mastodon.help/instances/en

By default, Mastodon servers can access all other servers. It's just the admin who can decide to block other instances on a blacklist for all the users on the instance they use. You can also just block yourself instances or users if you want. As long you don't take a political, shitposting, or fascist one which are often blocked on a lot of instances.