They would be more suitable with koi of that size in such a volume of water, especially if you had any way to view from the side also, they can be hard to see from above.
My way of sexing RTC has not been confirmed or validated by science or high level experts and is only doable for larger RTC, like 2'+ but you can find this info here: https://www.monsterfishkeepers.com/forums/posts/7886697