The "optimal" level to merge units whos skill changes after CC is 30CC20
If the unit does not get a new skill after CC just level 10 (not CC'd or anything).

For your Iris it would be 30CC20.

Having said that, I used two Iris's until I got Fedora and Camilla. Now I use those 3. I hated using duplicate units, but honestly, Iris is way better than the silver healers, though you absolutely want to hold on to a low cost silver healer. Some maps need to to bring out a healer quickly, and silver units are easy to CR as well as being cheap to deploy.

Keep in mind some maps will require 3 healers... and I remember needing 4 in one or two maps, (though I wouldn't worry about those fringe cases). Honestly, Until you pick up another healer (Camilla, Fedora or if the trading post starts rotating again Chydis) I'd rock both Iris's, and a silver of your choice.