I have been getting mixed reports. If I want to breed longfins, is it better to breed a longfin with a longfin or a longfin to a regular koi? I will get a certin % of ugly mutts either way, but which will give me the highest number of longfins?
To get the highest number of lonfins breed like to like. There are several different styles of longfins with very different body types - so make sure all the parents have a similar overall look. Cull the offspring with short fins as feeders or fertilizer.
One reason longfins 'get no respect' from folks who are serious about showing is that ther is not yet an established ideal conformation (body shape) or cut-off on minimum acceptable fin length. Basically the folks who breed them don't always care about producing consistent quality. The real hacks pass off the offspring with more normal fins as standard koi - which they are not (since the conformation is often wrong and because their offspring may have elongated fins).
Longfins are often bred to standard koi in order to get better colors and patterns. At least from what I have seen, longfin fry have longer fins even at 1" in size.
If you are breeding to achieve the highest percentage of longfin, then breed longfin to longfin. If you are breeding to improve the quality of your stock (except finnage), then breed to a quality standard koi and you will still produce a fair percentage of longfin offspring.