According to a study companion to NDdP by Rachel Killick (Glasgow Introductory Guides to French Literature no. 25, ISBN 0 85261 395 4), a 'Jehan Frollo' appeared in the Comptes de la Prévôté as an executed murderer (a career path which one has a nasty feeling our Jehan might have taken, had he lived, being somewhat Villon-ous, if you'll pardon the expression!). He also took the name 'Claude Frollo' from a lost of property owners in Du Breul's 1612 Le Théâtre des antiquités de Paris and made them into brothers. He may also have been influenced by the recent (1829-30) scandal of a young priest, the Abbé Louis-Denis Frilay, who was sentenced to hard labour for life for stabbing the husband of a parishioner with whom he was having an affair. (The case also influenced Stendhal's Le Rouge et le Noir.)
There was a real poet called Pierre Gringoire, but his dates were 1475-1538, making him just 7, not 23, at the time of the story. Whether Hugo knew this is a moot point: Gringoire was a fairly obscure figure, and it's just as possible that he got the dates wrong by accident as that he deliberately changed them to suit his story.
He accurately reflects the mediæval habit of naming people after saints' days and religious feasts near their birthdays. Pâquette must have been born on or near Easter Day; her daughter Agnès/Esméralda was born on St Paula's day (19 January), but that is also close to St Agnes's day (21 January), so her mother must have preferred that name or perhaps had her baptised on that date; and Quasimodo is named for the day on which Claude adopted him, Quasimodo Sunday (although he has an official birthday at Martinmas – 11 November – as they must have worked out he was about 4-and-a-half when he was found).
Phœbus as a first name, besides its mythological symbolism, probably owes something to Gaston Phébus, Count of Foix, a flamboyant mediæval nobleman devoted to "arms, love and hunting".