Not entirely. While you can interact with content from other social platforms like Mastodon and PeerTube from Lemmy, federation is best experienced when you utilize platforms with similar use cases.
For example, the link aggregators (Reddit-likes) on the fediverse are Lemmy, PieFed, Mbin, and until recently, Lotide. Each of these platforms have similar looks and purpose with slight differentiation. While you can communicate with video streaming or microblogging like PeerTube and Mastodon respectively, it's best to stay within the link aggregators and make a new account for those purposes.
To give an example with microblogging on the Fediverse, there's also a number of options. Biggest one is Mastodon, but others include Misskey and it's endless forks, Smithereen, Pleroma/Akkoma, Mitra, Threads, etc. all with their own interesting quirks. While they can interact with Lemmy and whatnot, their best interactions are going to be with similar-purposed platforms.
Immediate thought that comes to mind is that you can follow the Lemmy and Pixelfed accounts from your Mastodon, which would give you some ease in accessing their posts via Mastodon, and then boost the posts from there.
Hope that helps a bit, tbf cross-platform interactions like this are something I have very limited experience with.