First, All Paragraphs affect Paragraphs, i'm not sure if i understand your issue.
Let's say you have two paragraph.
One with a class "style", another one with no class.
Now you want to add a color on the Style class paragraphe like blue.
But your client wants all paragraphs red and bold.
So you go to the paragraph without class and you go to "All paragraph"
Oops something went wrong. It didn't applied the color to all paragraphs. Why ? Because you already styled one with the color blue. If you remove the style on the styled class, red will now apply.
It's because css in general and so webflow prioritise the style on classed element.
Another way to style text is simply to add a class on your body and define your font size, color and stuff. It's probably the best way because it will keep those style on all your elements.
Keep in mind that if you still have custom styles on classes, the values wont be inherited form the parent.
Inherited values are orange in webflow, while custom are blue.
About Rich text, you should definitely use them when you have more than one paragraph, and mostly when your client use cms.
To style rich text you should take a look at that
Hope it will help