There isn't, unfortunately. Global shows you all of the Fediverse, not just Pixelfed. A global pixelfed feed was a planned feature at one point but I'm not sure of its status.
For now, it's really hard to find pixelfed users on other servers because they get drowned out by all the Mastodon traffic.
It just depends on how you want to use your account. I can see wanting to follow PeerTube or Loops accounts from Pixelfed. I follow a few Mastodon users who just post photos.
The default behaviour is to only show content with attached media but you can change that in settings too. So you can use it as a microblogging service if you just like the look and feel of Pixelfed.
I'm not sure. I don't think I've ever seen a Lemmy post there, though I haven't spent a ton of time on that TL. It might be different because all Lemmy posts are posted to groups which aren't supported by Pixelfed yet. That's just a guess though.
Yeah I don't think pixelfed global supports Lemmy -- I've seen a couple image posts on Lemmy in the past 10 minutes that didn't show up on pixelfed global. So I guess they miss out on our memes and shitposts lel.