Just that Chopper wants to cure every disease, and if he had access to paramecia devil fruit user for a captain, when he's invented rumble balls for himself, there is a possibility he would try making rumble balls for paramecia types. (it would fail tho, Luffy being more rubber just meant his body physically couldn't support his own weight and he was just a wet noodle for three minutes[Luffy thought it was hilarious, Chopper cried])
And, theoretically, he also had access to a Logia when he met Ace. And if he was working on a paramecia rumble ball, he might as well work on a Logia rumble ball also Just In Case.
And Ace would, in the situation, maybe do whatever Chopper said because that was an entire Baby right there and 'aw Luffy remember when you were this small?'
But he took a rumble ball, and his fire just started burning so hot he melted through the ground and everyone was panicking until it wore off because a decent chunk of the desert they were standing on is now glass.
And thats sort of a success, so Chopper painted a few Logia rumble balls red and replaced a few of the beads on Ace's ugly ass necklace in case of emergencies and Ace went along with it because Chopper is a sweet baby
And guess what! Impel Down let him keep his ugly ass necklace! And you know what's really helpful when you're breaking out of Impel Down and facing an Admiral made of Magma? Being hot enough to melt that guy out of a solid and into a ver awkward puddle that he can't quite get out of because the consistency isn't something he's ever had to manipulate before