For business casual and perhaps for more formal business occasions.
Which should I choose, flat or pleated slacks?
I'd like to have a good set of rules when one is more appropriate than the other.
There are no rules.
Neither is more appropriate than the other. Plain front pants are "in", pleated pants are "out."
But fashion is silly, and often ugly. Pleated front pants offer practical advantages compared to plain front pants. They can flatter any build, are more comfortable, and will often even wear better. Plain front pants have plain fronts.
But to obtain these advantages you must wear them properly. I.e., non-fashionably. They must have a high enough rise, be worn loosely enough, and be properly cut and made, which usually costs more since all the cheap stuff is poured into fashion merchandise.