Are C600 cover control kits interchangeable with C400 cover control kits?

No.  Use C600 style cover control kits for the Box 1 non-combination starter.  Use C400GK style for Type 1 non-combination starters larger than Box 1.  Use C400T style for Type 1 combination starters and all Type 3R, 4X, 12 cover control.  See Eaton catalog for details - Volume 10 TAB 2