Ian,
most modern CFS2 scenery (including ALL of Achim's) uses the GSL layer for buildings, etc, this is so they are destructible, the whole point, no? GSL is not compatible with FSanything, that being said, older CFS2 scenery that uses API buildings should be fine, but I'm sure you know that already. Comes downtoo the fact that CFS2 had to wait for GSL placement tools to be developed, until then it used the same scenery tools as FS. So, hence the older stuff should work better than the newer.
As you also know, CFS2 doesn't use AFCADs, so they would have to be created by FS user to suit. And again, much to my dismay years ago, CFS2 doesn't do seasons.
Jamie