Learn Dutch: De kus was innig. - The kiss was intimate.

Both translate to "intimate", but they have slightly different nuances. Innig specifically describes deep, heartfelt, and genuine emotional closeness or affection (often used with hugs, sincere condolences, or kisses). Intiem is broader; it can describe a cozy and private atmosphere, a very close friendship, or physical and romantic intimacy.

Since this lesson covers the -de suffix to make nouns (like liefde), can I turn innig into innigde?

No, you cannot. The -de (or -te) suffix is generally reserved for physical properties (like koud -> koude, diep -> diepte) and a few historical, fixed emotional concepts like lief -> liefde. To turn innig into an abstract noun meaning "intimacy" or "fervor", you use the standard suffix -heid, resulting in innigheid.

How do I make de kus plural, and is there a vocabulary trap to watch out for?

The plural of de kus is de kussen (the kisses). The trap is that there is also the noun het kussen, which means "the pillow". Fortunately, the plural for pillows is de kussens, so they differ slightly in the plural form. Just keep in mind that the infinitive verb "to kiss" is also kussen!