Catfish usually mate inside of caves, and it's quite different than Koi. they snuggle more than rape if that makes sense. The male defends a nest until a female finds a suitable male, she will come in and lay the eggs in a large mass, the male will fert. the eggs then chase the male out and to find a new home. She will then stand guard of the eggs. depending on the species the male may stay close by and help or may go on to find a new home. I've never seen a catfish poke a hole in a liner and I've seen many mixed in with koi. I would advise betting a catfish that's large enough the KOI can't eat it. The catfish will pass the koi up at some point in size. As long as they catfish is fed well they usually really don't bother the fish that much, Now they are opportunist feeders so it's VERY possible for them to eat a small goldfish here and there.
If you want one that will show up, Albino channel cats show up good, get very tame, and can handle any temp Koi can. BUT they do get extra large.
I would like to take this time and mention one thing that some people don't realise. Catfish DO NOT clean out fish poop and left over gunk in the bottom of the pond. They eat insects, small crustaceans, small fish, and fish food. Plecostomus, also don't eat fish poop and gunk on the bottom but they do eat dying plant matter dead fish and algae. Pleco's can not survive once the water reaches 50 degrees. In fact they should be removed out of a pond well before the temp gets that low. They too get fairly large reaching over 30" long so housing them indoors for the winter once they get large does require a large body of water or fish tank. But IMO, they are one of the better pond scavengers during the regular growing season of ponds if proper care can be given to them.
