Thanks for the info about the Charis fonts. I downloaded the 'tuned' Sil fonts, and found that the Type Tuner was a great resource for selecting the small cap fonts. However, the problem with the Sil SC fonts are they don't conform well to what typographers seem to define as a well-formed small cap font.
"Generally speaking, small caps are about as tall as the font’s x-height. Look, for instance, at Minion Pro’s lower case m compared to a small cap Minion Pro m; it’s marginally taller than the lowercase m and the font’s x-height. Other typefaces have small caps that are the same height as the x-height, while others still stand a little shorter."
Correctly-formed small caps have the "capital" letters about the same size as a normal font capitals, and the "small case" letters are about a tall as the normal font lower case but in capitals.
My experience with old books from the late 18th and early19th century, is that these small caps fonts conform to the above, and in order to emulate as far as possible those typefaces in my epubs, I don't believe that the SIL small caps fonts are suitable because the lower case letters are too tall.
The ITC minion pro SC fonts would be ideal but are commercial fonts. The Garava sc font I mentioned is not much better that the SIL sc font but I modified it, with permission, to provide a 'petite' caps font with larger upper case letters. I probably went too far with this font though because the upper case letters are a bit too tall, but I like it better that way. I agree though that it is not ideal for epubs because it's a little hard to read (although no one should make a large section of text as small case).