Woo, I recently got a book about this stuff.
I would agree that Otho seems like a Mirmillo to me. Allegedly it means "moray eel" and they were considered to be the fish of the arena, often matched with the Retiarius as the fisherman with his net. And Otho has the two fish on his shield, just like the Mirmillo in my book.
It seems like Marcus is the only true Retiarius in the game, because one of their features was that they were the only ones of the major gladiator categories to not wear a helmet. But it seems fair to me to say that all the people with a trident and net are a type of Retiarius. Which would then include Mago and, as far as I can tell, Leika.
A Laquearius apparently fought with a lasso or long rope to ensnare opponents - sounds a little like the style of Zahra to me. Even though Zahra's combo of whip and halberd doesn't seem like it would work in real life.
But it makes sense that mostly Ludus Magnus gladiators match up with historical categories, since they are the most historically and orthodoxy based school.