There have been, in the past at least, a very few exclusions and limitations on the dining plan. But the list was always small and applied almost universally to 3rd party restaurants and DxDP appetizers. In general, it's a safe assumption that if it's on the appropriate section of the menu (as a single-serving item) then it can be had as a dining plan entitlement.
Most locations will allow two guests to pool entitlements for a "for two" item. Some will allow a single guest to get the "for two" item with a single entitlement. Some do not allow it at all. Reports vary, so it's often a location/date/server issue.