Fediverse only, I use Fedilab and Jerboa; on the desktop, tut.
I do follow Pixelfed accounts from Fedilab, and that works well enough. I have a Peertube account but almost never use it, nor follow any account there.
I think an app could probably do both Lemmy and Mastodon reasonably well; I only wonder how communities would represent alongside boost streams. The biggest problem for me, by far, is finding and joining communities; my subscribe list remains both small, and static; on Reddit, it was large, and constantly growing. And I wish hashtags worked better on Mastodon. Fediverse apps kinda suck on the finding and filtering side. If you could fix that, it'd be a greater killer feature than simple integration, which Fedilab is already pretty decent at.