Ranchu-Specific Spawning Issues

I was left with a lot of questions about my black Ranchu fry. Most centered around culling… I was not really sure what to look for early on. Here are some of the things I’m still trying to sort out:

• Backs: some of the Ranchu fry had really weird backs. In particular, some were what I would call swaybacked. I wasn’t sure whether to cull these or not. Still am not. As the fish have matured and their backs have become more arched many of these fish have improved dramatically. But there are also other fish that had more "normal" backs that have also developed well.
• Web tails: I had a lot of tripod tails which I culled. But I also got a lot of fry with partial tripod tails. Specifically fish with only the top ray or two of the caudal fin joined. I found that over time these improved a bit; the fins on some have become better divided as they mature. So I no longer feel I can automatically cull these very mildly web-tailed fish.
• Wen development: I am not sure what the fish need to have good headgrowth development. I have fish in one tank with good headgrowth progress and fish in another with none, yet both are receiving the same food. Weird.
• Black color: the black Ranchu have the black color in the very outer layer covering their scales. So the underlying scales are not black but some other color. I have had a lot of trouble getting this black covering to develop; at 6-months the darkest young fish look "sooty" but not black. My best guess is that sunlight and algae covered tank or pool walls help. This is based on noting that I have gotten the best coloration in the brightest lit tanks with the most algae growth.
• Which to breed: since I lost the original female, I plan to breed one or more of the young fish back to their dad. I find I have no "ideal" fish to use. All the fish have either a tail or a back defect. So which do I go with? If I can stomach the work, maybe I’ll try both and see what happens.