i saw it on the wiki page that they have qualities (normal, silver, gold and iridium) like any other fish. but because they can only be caught once, does that mean if i get a gold rank, i would never be able to try again for the iridium ones?
You are correct. As of the 1.4 update, all legendary fish may only be caught once. Prior to that, the Legend was able to be obtained multiple times, but that bug has been patched.
This updated kinda messed up with my collection, as I only saved the Legend since I could catch the others any time that I wished.