Hi Fox, the added advantage to the RACI finders is that the image is right side up - I used to have a straight through finder that was upside down and left to right. Trying to work out which way to slew in the dark was too much for me, these make it easier
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There are no stoopid questions, none of this information is intrinsic so ask away
. The "5x rule" takes pixel size, focal length etc into account and means that the actual image size for a particular OTA should be the same, regardless of which camera you use if you stick with the 5x rule. A camera with a bigger pixel size needs a bigger image across its sensor (so a higher f/num) to give the same number of pixels "on target". The maths describing the relationship is attached.
The concept is better explained in Steve's video entitled "Which Barlow?" (scroll to the bottom of the page).
http://planetaryimagingtutorials.com/getting-started/
Hope this helps,
Andrew