Hi Kuzko,
It sounds like this was a hypothetical question and I believe it has been answered. To try and quickly answer your question. It is unlawful in Mexico to keep any sport fish alive for ornamental purposes (aquarium) and it is unlawful to possess an undersize leopard shark in CA. Take a look at the Federal Lacey Act, this covers the regulations associated with importation of wildlife that is illegal. Now if this was taken commercially in Mexico and imported for the CA pet trade, there are potentially other issues.
If you would like to PM me, I can give you my number and we could talk about regulations.
Saltyhog,
CA defines it as "Take" not human consumption related to eating. Taking an undersize leopard shark for food or to put in an aquarium is the same. The possession is illegal not why you possess it. When things are commercialized (exchange of $) things get more complicated. "Take" is defined in the Fish and Game Code section 86 as to Hunt, pursue, catch, capture or kill or attempt to do so.
Another example of Take in Ca is you need a fishing license to take (catch and keep) a native snake, and there are limits on the number you can have. People are generally not eating snakes.
Jason