I'm a little confused here...
First you say you want a fish with a maximum lifespan because you hate things dying, then you say lifespan doesn't matter as long as they breed frequently and you can sell the offspring?
And you want to do this in a 10 gallon tank?
Here's my take on things... Death is a part of life. It happens. Some creatures have long lifespans (your koi, you) while some creatures have shorter lifespans. Everything dies, it's just a matter of lifespan.
That doesn't even take into account outside variables, such as environment, disease, or predation. And this is where your 10 gallon tank comes in: It's a smaller size, so it's more apt to have rapidly changing parameters than a larger tank, which may very quickly lead to non-ideal conditions for your wet pet. While it may not directly kill the fish, it can stress it to the point of compromising its immune system which can lead to infection and death.
As far as the breeding goes, you can rapidly overpopulate a 10 gallon tank with little babies. A 10 gallon tank is not something you'll be able to breed and sell fish out of.
If you want to have a long-living population of creatures, turn the 10 gallon into a shrimp tank. Get some Red Cherry Shrimp (RCS for short) and watch them go. You may start with 10 and very quickly have many, many more. They have a small bio load, so you should be able to keep water quality high.
