Do you think the Hightower has sole monopoly over the citadel? What if Sarella Sand with her knowledge gained from the Citadel begins a dornish version?

Depends what you mean by sole monopoly. The Hightowers are patrons of the Citadel, but they don’t control the Citadel, because the Citadel’s neutrality is key to its success. 

As to breakaway Citadels, I think the issue is that the main Citadel wouldn’t work with them or anyone who employed them, so there’s real downsides involved.