With Carcasonne crop circles expansion, which meeples count as followers?
There is a clarification on page 206 of the rules you link to.

Followers

Every normal follower, big follower, mayor, wagon, or phantom follower is just one follower.

Special figures

Builders, pigs, barns, and shepherds belong to individual players but are not followers."

So if you are adding a follower with crop circles you may add any of the followers. The only exception to this being a Mayor can only be placed as a Knight in a city.