There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
ninde
adjective. slender, slender, *thin
teren
adjective. slender, slender; [ᴱQ.] lissom, lithe
An adjective appearing as teren or terene “slender” in The Etymologies of the 1930s, derived from ᴹ✶terēn(ē) under the root ᴹ√TER “pierce” (Ety/TER). The Qenya Lexicon of the 1910s had ᴱQ. teren “lissom, lithe” under the early root ᴱ√TEŘE [TEÐE] also having to do with piercing, but Tolkien marked ᴱQ. teren with a “?” (QL/91).
terene
adjective. slender
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A word for “slender” in The Etymologies of the 1930s under the root ᴹ√NIN-DI “fragile, thin” (Ety/NIN-DI).
Neo-Quenya: For purposes of Neo-Quenya, I would also use this word for “✱thin” based on the meaning of its root.