Case by case, perhaps?

I imagine apart from the naming conventions required in public venues, that names and pet names used between friends, family, and spouses must have been fairly individualized.

For instance, the real Fitzwilliam family had (9) Williams in (12) successive generations, each of which were endowed with the title Lord Fitzwilliam. How the heck did they keep any of them straight? One Lord Fitzwilliam was named Charles William Wentworth-Fitzwilliam; my, how individuated he must have felt, sandwiched between two lords William Fitzwilliam.

Another stellar example of the problem names posted is that of Thomas de Grey:

(see [en.wikipedia.org])

This guy's legal name changed two times as an adult; he was also referred to as The Lord Grantham, although it's not clear why the article "the" was necessary (worth some research for amusement, I guess). A sketch of this gentleman by Dominique Ingres is entitled with his full name prior to a name change -- Lord Graham Thomas Philip Robinson -- of which the names Graham, Philip, and Robinson fell away at one time or another.

The only name which remained constant through his life was Thomas, which I hope like heck is what his immediate family and his friends called him.