You can definitely follow Twitter users. There's BirdsiteLive, a Twitter->Fediverse bridge which allows following Twitter accounts by reference from Mastodon (or any other Fediverse server). I'm using that to follow a small handful of users.
Nitter also supports RSS, so you can follow Twitter accounts in your feedreader.
What you can't do in either is reply back to Twitter profiles. BirdsiteLive sets the bridged toots (equivalent of tweets) as private, which on Mastodon renders as unsharable.
https://github.com/NicolasConstant/BirdsiteLive/blob/master/...
Nitter also supports RSS, so you can follow Twitter accounts in your feedreader.
What you can't do in either is reply back to Twitter profiles. BirdsiteLive sets the bridged toots (equivalent of tweets) as private, which on Mastodon renders as unsharable.