It's best to keep MC seperate from other fish. When fingerlings they can play nice with other fish, and I have seen setups where 250mm+ cod have been fine with larger fish. But two in the same tank, or with any other timid species will end badly. They are too much of a predatory fish, even if they can't eat another fish, they'll still smash it until it dies. As for breeding, they don't breed in enclosed waters, well there is very little evidence to say otherwise anyway. They need flowing water to do so. Which is why they thrive in rivers in Eastern Australia. I have thought about using a powerhead myself to see how they would react, but you would need a good size pond or tank so that both fish have a good area to themselves. That and you would also need to hope that one is a male and one is a female, as you can't sex them until they are pregnant (in which case you obviously know which is the female).
If you were to put one in a 240g tank, they could grow to a great length, but they are pretty shy and would hang around the same area of the tank for most of the daylight hours until they are comfortable. Which could take well over a year.
I have had my MC for over a year in a 4ft, 180litre setup with the intention of moving him to a much larger tank in the near future (when I say much larger I mean about a 400gal minimum). He has grown from barely a fingerling size to about 150mm in that time and will probably hit about 200mm before too long in which he will be a little too big for the tank. When I got him/her I also put in a Silver Perch of the same size. Only took about 3 weeks before the cod beat him up and he went belly up. And that was barely 60mm in length. I have a few friends and know of many others who have had bad experiences with them, losing cichlids, barramundi etc. I love Australian Natives, and thorougly enjoy my cod, but they are very hard to stock with other fish.
All the best