Since for some paragraph styles, there can be a separate sizes for Score & Parts, could I make an existing Custom Layout (a piano-vocal reduction) into a Part (to take advantage of the double font size possibilities) easily, without hurting any page layout things?
You can’t change the type of a layout, but you could apply the Part master page to your custom score layout.
Interesting, so it’s the Master page used, and not the Layout type, that determines what version of the font (score or part) is used?
It sort of depends what fonts you’re talking about and where. For instance, layouts use the bar number paragraph style set in Layout Options. Otherwise, the text not in the music that appears on the page (project title, composer, running header etc) comes from the master page. You could create completely separate paragraph styles to use in text frames in the Full score master page vs the Part master pages, if you e.g. want the title to be larger in the score than in the parts.
Of course, master pages can include “paragraph style overrides” - where you locally change text, such as its font size or alignment, to be different than set in the paragraph style. That applies in that text frame only, but if that text frame is on a master page, those changes get replicated on all pages that use the master page.
And one can add Custom Master Pages to use as alternates to the standard Master Pages if one wants to have both accessible in a document.