Its a flawed rule tbh. Its okay as a "general" guideline but tbh certain fish, having more around is better. Cories are one of them.
Id go with:
12-24 small tetras/rasboras OR 10-15 large tetras/rasboras (pick ONE species, or 6 individuals of 2 species)
20-25 pygmy cories OR 10-18 of the larger cories, dont mix pygmies with larger though. You could do more than this, but sounds like you want to keep fairly conservative, so ill stick with smaller numbers.
1-2 compatible "centerpiece" fish (optional)