You are lucky then, I have had single convicts which are almost as agressive as a breeding pair. I suspect that you may see some more agression as you begin to add more fish and territory starts to be smaller. If this starts to happen then the rams are not going to be able to withstand the convict (no way, no how), they would have a hard enough time staying alive from the cockatoos let alone a bigger, more agressive fish.
Also be aware that the cockatoos are harem breeders, not paired breeders. Like the beachboy song goes, "two girls for every guy", bare minimum at that, 3 females to every male is better. They are also alot more agressive at protecting their territory than rams, especially when breeding and as I said before, you are going to know about it once more fish begin to be added. The problem comes from alot of dwarves needing a large area for territory, some rams patrolling anywhere up to 2 feet square in the wild. This is a natural instinct which cannot be trained or adjusted out of the fish, it is inherent and nine times out of ten the rams are gunna get their asses kicked trying to protect their space. I would personally ditch the con and stick with a harem of cockatoos and maybe the rams as well if there is enough cover to hide in.
I am not trying to discourage you or be matter of fact either, I just don't want to see you blow alot of money and end up with a 20 gallon with one convict in it to show for your efforts.