A while back I was thinking about setting up a single user instance via mastohost. Turned out they use the newest Mastodon software (back then it was v3) and told me that because the newest version had dropped OStatus support, federation with gnusocial instances would not be possible. Don't know if that is still the case, but it was sufficient for me to look after an instance that, at least in principle and when everything works fine, supports ActivityPub and OStatus.