Not all the different services talk to each other in a way that you can use just 1 account for the entire fediverse. Lemmy, Piefed and Mastodon can all cross communicate (tho afaik Mastodon is one way only-you can post to Lemmy from Mastodon, but not Lemmy to Mastodon); but none of them communicate with Matrix. So you need at least 2 accounts if you wanted to use all of these services.