As loup said, the melting-pots are work in progress. While we may not have Romano-Alemmanic/Alemmano-Roman yet, we do have Romano-Gothic and Romano-Vandalic
Actually you may have melting-pots culture but it seems there's no melting pot event except the Roman-->Latin one for what I've seen in the event folder.
So except if you use the console or the ruler designer the melting pot culure will never appear for now.