Is there any reason to not allow some of the new "missing" generals to be available for Corps command (In other words do you guys want to avoid that since there may not be portraits available)?
Historically, Keyes (IV Corps) and Sumner (II Corps) were Corps commanders by the Spring of 1862, and if you were to allow this, should have high senorities. Butterfield (V Corps) and William F. Smith (VI Corps) were at this level by December of 1862. Had he not died right after 2nd Bull Run, Kearny probably would have also been at this level by December of '62. Couch (II Corps) and Crittenden (XXI Corps) were at the Corps level by the Spring of 1863.
As has been mentioned elsewhere, it would help to balance out the game if the Union player has to work his way through these leaders, maybe promoting them off to less crucial theaters of the war, before he can get the benefits of Reynolds, Hancock, etc.
Personally, I would even allow an Army level entry for Kearny....