I haven't found the right thread regarding this. In a recent MBR, the report on the I-Beam seatpost was very positive. It said that going that way you'd have to also get a Kore or SDG saddle. Does that mean any SDG including the famed (and very comfy) Bel-Aire? Can you combine the two successfully?

The Bel-Air is available in two models. The Bel-Air FX fits the I-beam seatpost and the Bel-Air RL fits standard seatposts. Most of SDG's saddles come in different versions for rails or I-beam. I found the Bel-Air XL with I-beam to be brutally unforgiving on a hardtail, but the Bel-Air RL is fine on my Orange Five.

Not quite. Most of the SDG saddles, INCLUDING the Belpair models, come in both normal, railed versions, AND in the I-beam versions. The two types aren't interchangable.