Nicknames are usually anachronistic. They were rarely used for nobles. One can use the title as a form of address as was often done, but then when their title changes, so does their form of address. Which also annoys the reader. *le sigh* Also, of course, some don't have a title if they are merely knights or a younger son.
It is complicated. I may refer to one as Thom just to avoid a complication but I will wince because it really is inaccurate. And I don't think Thomas Randolph would call his uncle 'Thomas the Red'. It really is one of the most persistent problems in my genre, how to tell one character from another. lol
This name thing even has been known to confuse historians. For four generations the head of the de Bruce family (somtime kings of Scots) were named Robert and a number of historians give the wrong place of birth from confusing son with father.