Your design is fine, but likely a costly design..... if you have 50K endpoints then 3 x 25K-CPPM is also OK... but as always corner-cases exist and you need to examine in detail the solution. If you tell me that the 50K devies connect of a short 30-minute window as this is a public venue then maybe its not the right solution.... if they are connecting in a 'typical' enterprise office over a few hours then your likely good..... but more details are needed..... I'd suggest leasing's with a ClearPass Specialist/Partner/Aruba-SE to get guidance.... it typically more than we can discuss over email on a forum.
For your second Q - Yes..... in a cluster you add ALL license on the PUB and the licenses are then available across the cluster.
HTH.