No, you did not make it up, Anya. That is the rule, except for some very ordinary words like "common" which grammarians still say can be commoner in the comparative and commonest in the superlative.
I never use these forms myself, and have suggested to all my students that they should not do so either, as most examiners, bosses etc. probably don't know that modern English grammar books still include them.
You are right too about adjectives ending on -y, which deals nicely with livelier, tidier, etc.
Orchids and other lovely plants that don’t need a lot of attention




