English has many anomalies and plural forms are often strange. That said, in the texts I've read, they did not add an 's' to kana, kanji, katakana or hiragana, but used them as their own plural forms.

I wouldn't have guessed that you were not a native speaker, though. You write well.

Just for information, be aware that you should not write 'softwares' or 'informations' either. Those aren't words.