Of course you can mix two styles, but it makes little sense as websockets does all the things REST can do, only faster and with state on the server. Clientside there is little difference between $.ajax(api, data) and conn.send(api, data). And it's just nicer to keep the api together serverside in (in my case) one SignalR controller instead of putting some parts in a Nancy module.

I do have some REST endpoints, i.e. for image upload and oauth, I did not know how to make that work otherwise on IE9.