does 并未 have to go with a pairing of words?
Your observation is correct. 并未 is formal and sounds natural only when it's followed by a two-character word (to clarify, these are not two-word sets).

It does sound too formal to use in spoken language and common/casual writing. You may want to use 并没有 in your diary or blog.

As for why it doesn't sound natural to pair 并未 with 1-character or 3-character words, I don't believe there is a simple grammatical rule. Linguists tend to explain that using either word formation or prosody rules. Common folks would attribute that to idioms and language sense.