I had no idea Lupin was a vegetarian, but that could be! Depends on how much credit you want to give to Gosho for keeping that in mind. (Granted,he probably did look into Lupin in his ideas to make Kaito a phantom thief, so it’s not a bad theory. I just tend to be skeptical if it’s a level of detail thing vs happy coincidence.)
Only thing about Kaito’s phobia is it’s not diet related, and it really does seem to be played up for laughs, which is… okay? But it’d be nicer if it were brought up more seriously from time to time, else it really does give the impression of a throwaway gag, rather than something meaningful to the character. Even as a joke parody sort of thing for the habit, it doesn’t work too well. (Though lord knows how many stories/theories/headanons there are for why Kaito does hate fish enough to where even a picture of them can give him a shock.)