Uuuh I’m not sure about that, but from what I read here and there, it’s not so obvious that Pixelfed allows federation ? According to @yalh here : [PixelFed] Photo sharing platform federated with ActivityPub - #7 by yalh76
It sounds like Pixelfed account will be followable from Mastodon, but not from another Pixelfed instance (did I understand that right ?)