In
http://www.carls-sims-3-guide.com/skills/ambitions/sculpting/You surmise:
There are two exceptions: The Immoderate Water Fountain and Sculptor's Gnome. Both of these will only appear at random when using the sculpt action. It's been suggested they may be the special sculptures you get as a result of having the Savvy Sculptor trait.
You're definitely correct that these appear only at random from Sculpt, but while I don't know about the Fountain, I did have the Sculptor's Gnome appear (in clay no less) from a Sim without the Savvy Sculptor trait this morning. (Sims 3 v 4.2.32.007001, base+WA+Ambitions, no mods other than hair and clothing). His traits were: Ambitious, Bookworm, Natural Cook, Green Thumb, Born Salesman. Probably worth updating, assuming you trust me! Granted, it's a nitpick.
Looking at the XML entry for the fountain, I suspect it isn't dependent upon Savvy Sculptor either, but I definitely can't verify that at this stage.
Love your Guide,
-Holmes