[@Sophrus]

An SAS is any polity (one that historically willingly joined the Imperium) that is a part of the Imperium but was allowed to keep their internal socio-economic and political structure. They are allowed a Senator (Protectorates only get a representative who doesn't get a vote) who can vote. They are also allowed to keep their own security forces (you could say military forces but the Imperial military is the only "true" armed force) and while not as capable as the Imperium's it would not be out of the ordinary for rival SAS's to have flashpoints and limited actions against each other due to anything from territorial claims to simple blood feuds. 

There is no set territory requirement, only that they do not exceed the maximum (at the start of the roleplay anyways, you may grow or shrink once it starts), and the population can vary as well. Though for population I would probably cap out in the ten's of billions or 100's billion depending on territorial make up. You could have a complete ecumenopolis with 100 billion people though that's not necassaril a good thing.... (aka it would be reliant on outside imports to survive). 

[@Drag]

There are various aliens humanoid or otherwise. The Imperium more or less draws the line on what can or could have been assimilated. It is not above exterminating aliens that pose a threat. However, aliens are rather small in political power and relative numbers to humanity since humans (and all their diverse appearances) are the dominant species of the Galaxy.