There are a few things that aren't a good reason for doing that.
1. The bigger discus will out compete the smaller discus in feedings which will make the smaller discus get weak and possibly get sick.
2. There will be aggression in the tank with the bigger discus to the smaller discus.
3. Before you add new discus to your 180 gallon tank you need to quarantine those fish for 6 to 8 weeks before introducing them into a community tank.
These are just a few suggestions.