I'm writing up a book using Adobe Caslon as the main body font, sized 11/14.5, and I want to use one font for figure captions, which will be placed in the margins, which will likely be sized around 8/11. Is there a sans-serif font that looks distinct enough, but does not clash with Caslon? I've been using Optima, but the results are debatable.
Also, I may like to use a different font for the cover page (very minimalist and mostly blank, except for title and name), but I'm worried about introducing a third font. What is the rule about that? If it is appropriate, can you make a suggestion to go with the sans-serif?
I could do the cover in Caslon, but I'm not a huge fan of Caslon at large sizes.