You can follow Lemmy communities in Mastodon by putting the "https://lemmy.world/c/fediverse" in the search box, hitting enter, the community comes up as a group in the search results, then you hit the follow button, and all posts will be sent to your home feed, it's not the greatest interface between the two, but you can post, and reply, like, follow, all that from and to a Lemmy community/user/post
No idea about Kbin, or Pixelfed as I haven't checked them out yet