It's totally possible, it works really well.
It's so seamless that most people don't even realise they are looking at other platforms! 😁
For example, if you click on @theatticdwellers you will see the profile of a PeerTube account which you can follow by just clicking on "follow". And the posts in that profile are PeerTube videos, which you can interact with by replying to them.
Another example, @Iancylkowski is a Pixelfed account. Click "Follow" to follow it.