Well, I guess it depends on the species. And yeah, they max out smaller than I was thinking. I think I was thinking of channel or something...
Anyways, you could keep one or two in a 250g pond, you'd just need some pretty good filtration.
As far as the fins piercing the liner, I wouldn't worry about it. If it does worry you, you can get a cheap 20mil liner on ebay for like $60. Put that inside the 15mil liner, and you'll definitely have no problems.