But I'm sure Lemmy/Mbin probably won't federate with Pixelfed, or Peertube.
Technically, they do! It's not fully-integrated yet so the process is a bit hackish, but because the ActivityPub protocol works mostly the same on the backend, content from any of these platforms can appear on the others without much issue.
The biggest issue just has to do with the way you actually interact with each platform may not translate well on another compatible platform. For example, Pixelfed typically nests replies to comments only one layer deep, whereas Lemmy or Mbin will nest many layers deep. The actual nesting still works just fine when you view Pixelfed comments on Lemmy, and if you join in the conversation and reply in the middle of the thread, your post will nest properly for you and will show inline as normal for Pixelfed users. So in some instances, you may need to know a bit about what platform you're posting to, on top of the platform you're posting from.
There's other quirks like Mastodon replies typically being preceded with @ing the user they're replying to, which isn't a thing on the Lemmy side of things. It doesn't really change the way posting works, and you can choose to not do it and things will still work fine.