Fishman Types

I believe one should have to post what kind of fish they are representing if they want to be a fishman, their features should be similar to such fish's appearance and no cross breeding fish, people have to stick to one kind.

Might want to read this:

http://onepiece.wikia.com/wiki/Fishmen

Crossbreeding is possible amongst the fishmen, wouldn't be surprised if this resulted in new fish.

"So far, all known fishmen fish-halfs are based on real-life fish species and not on any of Oda's made up species." There are no half tiger fish, half swordfish species. Since there are no fishwomen, only mermaids, their children would usually resemble that of the father's species or at least this can be assumed since there aren't different types of mermaids.