Daniel’s summarised it pretty well: one set of defaults for Score layouts and another set of defaults for Part layouts.
On your earlier point about Dorico letting you set 7.4mm as the default staff size, Dorico will in fact let you set any size you like as the default staff size. There’s a drop down list of “Rastral” sizes, which match the sizes in traditional hand copying rasters (which were a device for drawing five-line staves on paper), but underneath the dropdown list is a box for Custom Space Size. If you type 1.75mm into that, you’ll get a staff size of 7.5mm (as the Space Size refers to the gap between any two adjacent staff lines, meaning that the Staff Size is the Space Size multiplied by four).
I can have multiple layouts. Each layout can have one or more players and each player can play one or more instruments.
When I bring up the Layout Options dialog, I can see the various layouts in the current score on the right. I can select a layout and set various options for it, including the Page Size and Space Size. If I apply any changes, they become effective for the selected layout.
The defaults are a bit trickier.
If I press Save as Default, the current values become the default for a layout of a specific type: full or part. How does one designate a layout as full or part? As best I can tell, you do this by selecting a page template. You can have many page templates, but each begins life as a copy of the factory full score or part page template. Any page template copied from the factory full score is a full score template; similarly, any page template copied from the factor part template is a part template. The page template type appears to determine the type of the layout: full or part.
If I create a new document (not from a template) and add a player (and instrument), one full score layout and one part layout are created. The full score layout settings will be whatever was last set as the default for any layout associated with a full score (by way of its page template setting). The part layout settings are handled equivalently.
No matter how many layouts you have, there appear to be only two default layouts. If you have two different layouts for parts (through association with a part page template) and press the “Save as Default” for each, the last one chosen wins.
I think you said all this in fewer words—I had to work out the full details for your description to make sense to me.
When you tell Dorico to create all missing Layouts, they are automatically what they should be i.e. a score will use the Full Score default Layout Options and parts will use the Part default.
If you create a Layout yourself by doing this:
You’ll see that you can choose what type of Layout it is so that the default of that type is applied.
I’m not sure how to change a Layout from a Full Score to a Part
You can change the Page Template that Layouts use: