There are other fish you could keep in a 30 ltr (7 US gallon tank): microrasboras, male endlers, heterandria formosa, sparkling gouramis and ember tetras are just a few that spring to mind.
But goldfish are certainly not among them. They grow really fast if healthy- and if they're not growing fast, something is not right. As the goldfish grow, they would fill the tank with ammonia which would poison both them and the neons. Not to mention that goldfish and neons have different temperature requirements. And that grown goldfish would eat neons.