New user here… really enjoying Scrivener so far (which hasn’t been too far yet) but I’d really like to be able to write in what’s presented in the Novel Format template as Standard Manuscript or Paperback Novel styles.
That’s entirely impossible, and not the kind of program Scrivener is. It is designed from the ground up to separate the writing environment from whatever output formatting one needs in the end. In this way we can do more than the editor even allows, such as multi-column layouts, footnotes, etc. And if you find an 18pt font easier to look at all hours of the day, it doesn’t really matter since the output will go on using 12pt standard (or whatever it is designed to do).
Closest you could get would be to compile some sample material with all forms of formatting you anticipate needing to RTF, import that back into your Binder, and then use that to update your stylesheet. Refer to Redefine a Style, in §15.6.3, Using and Managing Styles of the user manual PDF, for further information. The previous subsection on creating new styles will be of use if you aren’t familiar with the style setup panel.
For your body text I was strongly recommend not using a style. Instead just put your cursor into the imported RTF file where there is body text, and open the Editing: Formatting preference tab. Click the Use Formatting in Current Editor button to bring the paragraph settings in. Now newly created documents will look like this, and old documents can be updated with Documents ▸ Convert ▸ Text to Default Formatting….
Also note the Set Styles Defaults… button at the bottom of that preference pane. Once you get your styles set up the way you want, you can make them the global default for new projects.
Indeed, while Ioa is absolutely right that it’s impossible to write using one of the Compile formats (since they are like formatting brushes applied to your manuscript during export or print), popcornflix’s advice is good if you just want to get your writing environment looking like a paperback. You would want to go into File > Page Setup and customise the page size so that it is smaller than the standard A4 or Letter size, use page view, and then pick a default font (via Preferences) and line spacing that emulates a book. The default Palatino is pretty good for this, though.