Was the fish guy right to recommend this?

The rams are quite peaceful, but when they are breeding they can get aggressive to the point of stressing out your other fish, possibly even to death. The biggest problem would be the corys. The ram pair could live in the 35L, but they need pristine water to thrive. I think you're better off with keeping your gourami. rams only get to 3-4" maximum, so they don't get as large as you might have thought. It's ultimately your decision.