A fast death at the hands of a predator is preferable to a slow death at the hands of an irresponsible keeper IMO. Neither the 10 gallon or the 40 gallon will be suitable for your comet long term. The 30 gallon per fish rule does not take into account the fact that goldfish are social creatures by nature. You're not doing the fish any favors with your plan, and in reality, it's only prolonging the fish's suffering.
I agree. The smallest tank I would put a single goldfish in would be at least 75 gallons (preferably much more due to the rediculous amount of waste they produce). And I also agree that the "1" per gal." rule is complete bs. According to that rule, I could keep an adult arapaima in a 150 gallon...Actually, the one inch per gallon rule is complete bull dookey. It's clear that you've got your mind made up, and will happily disregard all the advice that doesn't agree with what you want to do. Given that the longest lived comet clocks in at over 40 years, 10 is no big accomplishment. I wish your fish the best of luck, as it will certainly need it.
For a fish that can get to over 12", I fail to see how a smaller tank could possibly be considered adequate, especially when the fish in question is among the most waste-producing fish kept in the hobby (relative to size). They can be kept in less, but doing so would likely result in a shortened lifespan, and possibly stunting depending on whether the owner is able to keep the water parameters under control (which can be very difficult with smaller tanks).Dag! 75 gallons for a single goldfish! That's palatial! I agree that comets aren't the best choice for indoor goldfish, but I think that 75 gallons would be more of an ideal situation than what the average goldfish enthusiast could or would provide.
Take a look at Flaringshutter's tanks with her comets. They are beautiful. I think she has a gallery on her home page. She knows goldfish (especially comets) and has great results with them.