Spensar, I saw this and thought you had it all under control
Your plan sounds good, using anchor cups that will allow you to fully secure the ladder via the anchor wedges. 2' x2' x 5" should be sufficient and do keep the bottom holes of the cups 'open' so that they can drain. Bonding them to the pool walls is also a very good idea, let's keep the pool and equipment safe for anyone using it!
As you have a small area to do, using Sac-crete or whatever other ready to mix 'crete should do what you want it to do - it's not worth the expense of having a 'ready mix' truck come in for so small an area (they charge extra if you don't get at least 3 yards ) If you want to put a few metal coat hangers in the middle of the pour, for reinforcement, that'd be good, though I doubt it's necessary.

Should you need any other help, or clarification on what I've said - please feel free to ask -- good luck with this project and HAPPY SWIMMING