A 30 gallon is going to be too small for a convict with running the risk of stunting and then death. Convicts will also kill anything you put in with them. I recommend getting rid of them when they are small, because no one will take them when they get older. They might seem peacful a few days and then they can turn overnight.
I guess you could do a single convict in a 30 gallon, but it's always nice to have more room if you ever want to add more fish or once he gets to his maximum size. How large is he right now? Are you planning on getting any tank mates?
I'm not saying don't put him in, i'm just saying think forward a little. Forward thinking is one of the most important parts of the hobby imo. If you don't plan on puting another big fish in the tank, go ahead and set it up