I don't see the characins and the cats working out, the cats will continue to grow, already having a huge size advantage doesn't help, and the fact that atf and dorado are elongated only makes them easier to eat. if you want to add something add dithers for now. I say focus on trying to catch the dorado up. Cause they'll probably slow down in the 400 and then get a growth spurt in the 1700, I say this because a lot of people report them slowing down in smaller set ups but growing fast again once put into an upgrade. The dorado will probably slow down around a 12" and then grow to 18-20" in the second year. And still will be edible to the cats, I don't think 5-6 atf and a dorado at those sizes would be happy all together in a 400.