I never had any luck with christmas cactus in full sun, those plants get fried to a crisp with that kind of exposure. As for humidity, we average about 80%, nightly to 90%, lots of fog. No problem whatsoever for the succulents I grow. There has to be some overlap between orchids and succulents (especially indoors) but I would guess it's the epiphytic lifestyle, rather than sun or humidity preference per se, that would indicate compatibility among succulents. That said, a surprising number of succulents can grow hydroponically, in a clay pebble type medium.