I've never used the Elation Satura so can't answer which is better. My venue owns a number of Clay Paky Alpha Spot HPE 300s and they are rock solid, quiet, and reliable. The Elation appears to be LED with CMY so probably less expensive to operate in the long run, assuming the LED engine doesn't fail. On paper, the Alpha is twice as bright.
1. Can I use the CK HPE 300 Spot on a conventionaletc standard clamp that the etc source fours use?
2. If I purchase the optional clamp by clay paky will this fit on a standard theatrestage pipe?
Most people prefer trigger clamps or half-couplers for MLs. There are several manufacturers. Most are designed to work with truss or pipe with a nominal 2" OD. Common stage battens are 1.5" SCH 40 which has an OD of about 1.9", so yes they will work. The Light Source TC Clamp is my favourite.
Pedantically speaking, the ETC clamp isn't rated for use on other than an ETCfixture.