Starting Grant as a 3-star army commander in Feb '62 makes him over-ranked by quite a margin. What does the Union player need Halleck for, if Grant can start with an army?
Grant was a brigadier when he fought the battle at Belmont in Nov '61. He was a brigadier when he took Henry and Donelson.
He was promoted to major general in Feb '62, when he appears in AACW.
Either make him available earlier, like October '61 as a brigadier, so the player can use him for some Belmont like actions, or wait and make him available in Feb '62 as a 2-star general, so he can become a corps commander under Halleck, which was exactly what he was.