Is it within WordPress guidelines to update another plugin’s database fields from my own plugin? [closed]

WordPress guidelines doesn’t restrict doing this. In fact, modifying database directly is no different from doing update_post_meta() or update_option() which are used widely everywhere.

However, modifying data of other plugins need to be done carefully because the data might be referred somewhere else.

I’d recommend using the plugins’ hooks to get/change the data instead of modifying it directly.