The caption weight seems appropriate for a 7.8 size text. But looking at your pdf I noticed that you've used, both in the regular and caption settings, the Light version, which seems very uneasy to read at that small size! So I would try first to typeset the entire thing with the Normal version before proceeding. (Chapparal light works very fine for titling, in bigger sizes). Anyway, I don't think there is a general "rule" for using the optical sizes with the exact points recommened by Adobe: I think they're more like suggestions.

Yes, I went with the Light version because it seems to me that it gives more contrast to heavier weights than the Normal does. At any rate, I tested the Light+Regular version - it looks fine in print.

I'm just wondering if it looked better with Light+Captions. I'm not sure whether there is any notable difference between 'regular' and 'caption' besides tracking, which is obvious. Maybe it's a bit "heavier". I thought maybe there are other subtle differences that I don't see, that make it more suitable for smaller sizes, or that only show in print.

I took a look close-up in photoshop, and it appears that the main difference is the tracking, but the letters are a tiny bit wider in light caption and a hair thicker (barely).

I think it was originally a Multiple Master font with weight and optical-size axes. Later they re-released it with a static selection of weights and optical sizes. These were probably just created by interpolation on the MM axes, so the variation seen between Light and Light Caption is from the optical size axis.

Your best bet is probably to print it on a good laser printer and see which your eye prefers (assuming this is a print project).

Personally I find Chaparral Light to have a lot less character than the Regular - the light has very little contrast and looks almost like a monoline font.