Not sure what you mean by legals? Calico bass, halibut, yes. If you fish for any US fish in Mexican waters, "that are out of season", such as Lingcod, or Sculpin, or any fish undersized, etc. you cannot fish US waters with those fish in your possesion. So no!
If it's a legal fish in both the US and Mexican waters, it makes no difference, "to my understanding". IE: you catch a yellowtail on a kelp paddy or at the Coronado Islands in Mexican waters, you can still fish La Jolla on your way home for either more yellowtail or any other species of fish, however, there is a size limit on tails in the US, so you must also follow those rules.
Isn't life for a fishermen complicated, gad so many rules, sizes, and seasons, etc?