You can ignore Cosmology of Mutazilites. Concentrate on their five points

(1) the unity of God; (2) divine justice; (3) the promise and the threat; (4) the intermediate position; and (5) the commanding of good and forbidding of evil.

that is the main thing. if you read Saeed Sheikh's mini book after reading in detail you ll be able to better understand and retain stuff. If you can cram things well, go straight to Saeed Sheikh's book.