To be honest, i wouldn't have euthanized him yet and IMO freezing wasn't the best way. That way is too slow and kind of torturous..once again my opinion. Clove Oil would have been A LOT better....anyway.... if you caught the Dropsy early enough (i'm not sure, you didn't mention how long he had it or when you noticed it) you probably could have tried to treat him and then atleast you would have known you tried to help him but couldn't. There are 2 kind of dropsy, contagious and non-contagious. Non-contagious happens when you over feed him to much and then you've got organ malfunction which causes the Dropsy. You should feed him one day and then don't feed him the next day, that way you can really decrease the chances of developing Dropsy. The contagious form is from a virus or infection but usually theres another problem involved before the dropsy sets in. Not sure what kind your Betta had but I THINK the more common one is the non-contagious kind therefore you should be okay putting in other fish