This is really interesting and a possibility! It might even be a fun headcanon to play with if you’re RPing or writing fanfic. I would want to sit down and watch the movie again to see what indicators Cassim gives of how much time has passed since he rose to prominence there, because I think only a couple years have passed since the first movie, and Cassim’s been gone since Aladdin was wee. (That’s not to say he left and immediately became the King of Thieves; Sa’luuk talks about a time before he came along, after all.) But things to look for would involve references to time passing in his dialogue. How does he seem to talk about his experiences with the 40 Thieves? Does it seem like he’s been there longer? What’s more, how do the other members of the band talk to or about him?
One thing we can probably be certain about is that even if it adds up, it probably wasn’t intentional on the writers’ parts, with regard to the first film and how it plays out. Remember that Return of Jafar was the first direct-to-video sequel they’d ever done, and it pretty much started the slew of video sequels we got throughout the 90’s and early 2000’s. Before the final script for the third movie got greenlit, the only certainty was they intended to focus on Aladdin and reveal a long lost relative. They originally intended to reveal Mozenrath was his brother, but pressure from the studio led them to distancing the final movie from the series because they thought people who did not follow the series would not buy it. (In the end it wouldn’t matter, since non-fans of the series bought it because Robin Williams was back.)
The DVD Fairy 