Yes I consider cardigans toppers! Like Rambling Ann, sometimes I use hats as a third piece. Or belts/sashes. Or my brooches, although they're obviously smaller. Now that I've taken up knitting, I'm playing with the idea of fingerless mitts of various lengths as a third piece. Like Traci, I haven't really been able to utilise vests, although in my head they'd still be a good summer option.
I don't follow a particular rule re: matching them to something else, although most of my jackets are in my preferred neutrals, just because I haven't come across many jackets in the colours I love. My cardigan collection is more colourful, although I like having options in my neutrals too, since I have a lot of colourful blouses. In addition to mixing neutrals with colours, I love to wear tonal variations in the same outfit, so I might combine a mauve cardigan with a blush blouse, for instance.
For summer I use either lightweight cotton or linen jackets or cotton/linen/silk cardigans. I don't run cold (I wish I did!), but I prefer to cover my arms from the sun with a layer of fabric, so I just deal with the extra warmth from the topper in exchange for not having to slather sunscreen over my arms everyday. I leave the topper open almost all of the time though, for breeziness around my torso!
The toppers I find easiest to style have a fairly defined waist and end somewhere between my natural waist and high hip. In high summer the waist definition isn't as necessary, since I wear a lot of a-line shapes up top to maximize those breezes; in those cases I like being able to button the topper right at the top, so that it makes an interesting line.
Other than that, I just incorporate them the same way I incorporate other separates!
