My father gathered many toona ciliata seeds, they grow rapidly, they all shot up and generally there would be a dominant tree for every pot and would grow massive compared to the others in the pot. As a bonsai Ive got a few going, the pros are they grow very quickly and can thicken up quite quickly in a pot, they seem to all develope a large tap root which elevates the tree slightly from the soil. As a rainforest tree their growth habit is upward, upward, upward. I have continuely chopped the crown off my bonsai's and they continue to develop another crown at the cut site, with little to no branch development. Maybe as they age they develop more outwardly, but for the moment they seem to only want to develop height.
I have grown alot of Red Cedar over the years but its not a plant I'd choose to grow as a bonsai. We have one about 1m in diameter at chest height on our property and many in the 10-15m range. Very few retain lower limbs that seem to drop easily and this, combined with large compound leaves, would be my main concerns. One other issue if growing near bushland would be the cedar tip moth thats larvae hollow out the trunk evidenced by an ooze on trunk. This is why you don't see Red Cedar grown successfully in monocultured plantations despite the timbers high value. They are pretty quick growers though and I'd like to see one in training as a bonsai. Good on you for giving them a go.