It is true that a majority of Morrighan are Gwerin, members of other races also serve the organization. Anyone that can hear the dragonspeech and dragonsongs can be selected for training. Not all cadets that enter training will graduate. Of those that do graduate, not all will impress a Ddraig.
Ddraig can mimic a human form much like an octopus can mimic its surroundings. However, the only humans it can emulate are the ones it has been psionically linked to at one point (imprinted on / flightbonded). So, yes, a Ddraig might appear as his or her current Morrighan or one from their past.
Ewwww, no! The Ddraig cannot interbreed with humans or human-like races. They do not find them remotely attractive and are physically incompatible.
The Ddraig do not keep slaves or serfs. There are various people that choose to serve in the aeries and citadels to make life easier for the Ddraig and the Morrighan. They are highly regarded members of the various households.
The sieges are vast colonies of dragons and bipeds. They have to be supported and maintained to perform at their peak. Since there is little arable land around most sieges, they do require that the lands and people they defend help support them. The content of the tithe varies based on the nation's wealth and major products. A tithe can consist of farm goods, livestock, and Morrighan candidates. It can also include crafts and other trade goods, grains, and seeds. The siege is not totally dependent on the tithes for support. The dragons hunt and, since they can bring down the megafauna, they share their kills with their Morrighan. The biped population also turn out crafts, farm and raise livestock if there is enough land.