@Ernest Bywater
Many style guides allow for the use of symbols like the percentage sign (%), the dollar sign ($), the ampersand (&), and others in the text of narrative while some even allow their use in dialogue. However, you should avoid using any of them in dialogue, and only use the dollar sign ($) in the narrative when showing an irregular amount of money. Using the words to show five thousand dollars in both the narrative and dialogue flows more smoothly to the reader than $5,000.00 does. Not only do the numbers and symbols stand out more than the words, thus being a possible visual disruption to the reader, they don't always convert over into other formats cleanly. This is due to many symbols being used for code commands in software languages. The symbol of the ampersand is used as the first character of a lot of HTML codes, so it can disrupt the correct conversion of the text into HTML.
Sorry, Ernest, but this guide is utter nonsense (I'm guessing it was written quite a while ago, back when the internet hadn't agreed on how to present characters online (the old PC vs Apple wars).
Using an ampersand only causes a problem when you submit your work to an html conversion (like the SOL forum), in which case you simply use the html ampersand command ("& amp;code;").
But, avoiding all symbols solely because you aren't smart enough to figure them out is questionable advice, at best, and is delusional thinking at worse.
I'm been using publishing characters, including accents in foreign languages, for years, both on SOL and in publishing (eBooks and Print), and have never had a problem other than forgetting to double check they appear correctly (Word routinely converts foreign text into another font which won't display the accent characters properly, so those sentences have to be reset to the correct Style Definition. However, one should always research how to represent special characters before they attempt to use them in fiction.