The French Peas were the only cast members who appeared in the flashback as characters distinct from their roles in the film's wrapper story. Perhaps they really weren't different characters after all: in the flashback, they're selling fish, and in modern times... they run a seafood restaurant!
They're descendants of the Ninevites. As for being French, they weren't popular among regions of the US for their refusal to get involved with the War Against Terror after 9/11. Also, Big Idea were fans of Monty Python, and the peas are basically John Cleese's French soldier from Holy Grail.
The Anachronism Stew is due to the Pirates telling the story to modern-day children, making it more familiar and relatable to them.
What Khalil meant by his uncle being hit by a fish was that he might have got The Slap of No Return and been killed. This is because part of the device that gives said slap is shaped like a fish.
