A syllable is a phonetic element of a word containing a vowel, take the word ‘containing’ you could break it down to ‘con-tain-ing’ and have a symbol for each part. In Chinese each word is just one syllable long, a monosyllable. What would polysyllabic be?

With many syllables?

Exactement.

And oligosyllabic would be with few syllables

It would be, but it’s not used much, people seem to work in terms of an opposition between the one and the many

Duosyllabic

It would be better to say word of two syllables on grounds of euphony. In general if you are going to make up a word you should use the adverbial form of the number, which would give disyllabic except people often seem to use bi after mono, monogamy bigamy monoplane biplane. Usually Latin numbers go with words of Latin derivation, so unilateral bilateral multilateral bicameral multinational, and Greek numbers with words of Greek derivation, tetrahedron, tetralogy, pentagon

Trisyllabic

Yes

Tetrasyllabic

Yes

Pentasyllabic hexasyllabic heptasyllabic oktasyllabic enasyllabic dekasyllabic hendekasyllabic dodekasyllabic

Exactly

Treiskaidekasyllabic tessareskaidekasyllabic pentekaidekasyllabic hekkaidekasyllabic heptakaidekasyllabic