Pixelfed used with a private account does not seem to communicate very well with Zap or Mastodon. A new post will show up for Zap & Mastodon followers, but their comments will apparently not leave their own servers. For instance, a comment posted from Zap will not appear anywhere, not in Pixelfed nor in Mastodon. Same goes if you try do do the same from Mastodon.
Things work a little better with public posts on Pixelfed. Comments will be shared between servers more efficiently, but not reactions (likes), i.e. reactions on Zap are not visible in Pixelfed.
I'm not sure that Pixelfed is completely ready for sharing privately with my friends & relatives, but it's a very nice project, that will only be improving in time. The UI is already very nice for photo sharing. If someone would work on a similar UI for a Zotiverse project, it would sure be nice... but, after having a quick look, I know that bootstrap based theming is way more complex than what HTML/CSS theming was 20 years ago, so I guess it would require lots of work.
Anyway, for the moment, if I want to share pictures of my kids with their grandparents, Zap will certainly be sufficient. Also, it allows me to do it privately and securely (I'm not that confident for "real" private communications using ActivityPub). I'll keep following Pixelfed's development, but will not launch a server for my friends & relatives for the moment.