Ada lets you focus a particular old exotic armour for a particular stat drop. (Example sunbracers for grenadier or paragon, etc) This lets you shift the preferred "old" exotic armour into the 3-stat system. She only lets you focus 20 times I think. Rahool is random. He's converting engrams (either by opening an engram randomly, or by selecting a particular one) under the new 3-stat system, but the type (gunner, brawler, etc) is random. The new exotic armour from Edge of Fate or Renegades you can't buy from Rahool until you've had one drop naturally. You cannot get them at all from Ada.