About the weight - with lightly rigged camera package you should be close to, but within the limits of you sled. The 20lbs limit is for everything you put on the sled - so the camera, all batteries, accessories etc. 30 lbs is maximum the arm can carry. So stick to the 20 lbs limit for safety. Atlas Orions are 5-7 lbs each, camera body around 7-8 lbs. But your issue will be power, as Alexa 35 needs 24V that Aero 30 doesn't provide. So you will need to run onboard battery for the camera, and that will of course increase the weight of camera package. Also keep in mind that you won't be able to use the batteries provided with the camera (B-mount 24V) for your sled (AB mount or V-mount, 12V), so make sure you can balance the camera with the batteries and counterweights you have access to. It would be best to test everything beforehand. Weigh the camera package + sled battery, then weigh the whole sled with the camera and batteries to check if you are withing limits of the sled and arm before proceeding. Also keep in mind that your options of extending the post for low or high shots will be limited if you are close to maximum weight capacity of your sled.
And make sure you are properly insured before putting the expensive af camera on your sled 🙂