Yes, in a realistic setting there'll be vehemently psychotic anti-human fishfolk ala Arlong. I just don't think Oda will show them and make them a major part of the plot.
Even so, just because something isn't realistic, doesn't mean it's bad writing. For more information, read Kafka and Gabriel Garcia Marquez. One Piece is FAR from realistic, yet it's written very well. It really depends on how it's handled. It makes sense that in Alabasta there would be people vehemently anti-royalty despite what one kid said. Yet we didn't see them.
Arlong was in the Sunny Pirates. He had the Sunny Pirate tattoo and was described as "let free" when Jinbe joined the Shichibukai ranks.
Of course Jinbe can't deny it. I can't deny that many Mexican-americans got to the U.S. illegally and adhere to stereotypying as a hispanic. That doesn't mean it's inherently true, and it doesn't mean I'm not going to get offended if someone tells me all Mexicans are X.
Oh man. Hello, there were people that were anti-royal, they were part of the revolutionary army, there were also people that believed that their king was a saint who did nothing wrong, you get that in nations. Everyone in the Sunny pirates was given that tattoo so that everyone was EQUAL, nobody could tell whether a fishman was a slave or not. Arlong could have joined after fisher toger left/died, we don't know. Then what are you getting at? There were psychotic fishman that went with Arlong, there will be psychotic fishman left on the island, do you get that? THERE WILL BE FISHMAN THAT HATE HUMANS AND WILL WANT THE BLOOD OF HUMANS, IT WILL NOT BE BAD WRITING ON ODA'S PART IF FISHMAN LIKE THIS ARE ON THE ISLAND, IT'LL ACTUALLY MAKE THE SITUATION ON THE ISLAND MORE INTERESTING. Is this getting through to you?