Hey Sylvia!
I'd say each part of the structure should repeat in the same way (rule for memorability) but every rule can tolerate a few exceptions (and what you say should always prevail on a mere count of syllables).
It's a rythmical thing. You can always tie 2 (or more) notes into just one, or divide a rhythmic value into several parts. The result is always a variation of the same rhythm, though. We just have to be careful about not writing a 'different' rhythm.
And of course not overdoing the trick. A good line can be 'reproduced' with a different number of syllables, but I'd say 1 or 2 (more or less) is a maximum (and should always depend on the tempo and how difficult it is to sing some particular words).
I've also noticed that the more songs I write, the less I have to ask myself the question. Of course, I always write against a pattern, so I just sing the line and 'see' if it works. it usually does, as if something in the brain had internalized the pattern. When it doesn't click, it's usually a case of too many fast syllables at one point.
hope some of it helps. just my 2 cents

Yann