AFAIK (and my knowledge of Italian history at this time is touch-n-go), Cosimo was OFFERED leadership, the famous comment about "some had in mind the glutton, some the innkeeper", because like so many factions offering a theoretical crown they thought Cosimo would be a figurehead who would be beholden to them out of gratitude.
BOTH Alessandro or Ippolito would be IMPOSED on Florence, albeit by a Florentine pope and coming from a Florentine family on a city that threw them out several times. Added to that, a marriage with Caterina is unlikely - she may be legitimate (as opposed to Alessandro or Ippolito) - but besides a tenuous connection to France, she is of no theoretical value, unless Ippolito decides to go reconquering Urbino. In all likelihood, Ippolito marries another Italian noblewoman (Farnese?) or Margarethe of Austria, who can bolster his regime. A possible double French match, Ippolito to Marie de Bourbon (he won't get one of François I's daughters, Henry Fitzroy had a better chance of that) and Caterina to the duc d'Orléans is on the cards, but Clement VII might see that as putting all his eggs in the French basket.
It might be interesting if after the Sack of Rome, when Clement is at least theoretically allied to Karl V, he dusts off Leo X's plan to annex Ferrara and parts of Emilia to the Medici, with imperial support, plus securing a marriage between Ippolito and Margarethe, while at the same time negotiating a marriage for Caterina with a Farnese (Margarethe's OTL husband) or even perhaps, he gets a match to the duke of Orléans for her.