From the Japanese songs I've listened to, I belive that you do not need to be that careful about the number of kanas in a line. You can always do two things:
- Sing the lyrics kana-by-kana or "the ordinary way". For example, 階段｛かいだん｝ could be sung as either just kai-dan or ka-i-da-n. The former would be 2 syllables and the latter would be 4 syllables.
- Give "unorthodox" furigana to words. I'll illustrate this by an example, which is from Morning Musume's 「時空｛とき｝を超え 宇宙｛そら｝を超え」:
時空｛とき｝を超え 宇宙｛そら｝を超え 結ばれる頃には
Note that 時空, 宇宙 and 地球 are usually pronounced じくう, うちゅう and ちきゅう, respectively, while とき, そら and ほし are respectively 時, 空 and 星. However, the songwriter may assign to words whatever reading they desire, even if it is not a valid reading of the word.
Sometimes this is used for special effect, but sometimes it's probably just to fit the number of syllables, such as this line in AKB48's 「桜の花びらたち」:
その背中に 夢の翼｛はね｝が 生えてる
I don't really see a reason why 翼 is not pronounced つばさ as it usually is, besides that the syllables would not fit...
Of course, if you are writing enka, you might need to be more careful about the number of syllables. But I suppose you are not into that, so you don't have to get mathematical about the number of syllables.