Brand new with C&C. If two cavalry units are in melee so in adjacent hexes is it still ok for a nearby infantry or artillery unit to use ranged fire at the enemy and thereby effectively firing into the melee?
CCN is not a tabletop game with "mixed" melee rules
Each battle in your turn is treated one at the time (except the special combined Attack rule).
And you fire only in the hex with the enemy.
And now (see my start sentence) in this hex is only one unit - the enemy.
Therefore you can also only hit this unit (and not another, friendly or enemy) unit in an adjacent hex.
As long as the infantry (a) has a clear line of sight to the target cavalry unit, and (b) is not in melee itself, it is free to shoot at the cavalry (or any other unit, for that matter.) Line-of-sight is blocked by other units, remember. The infantry fire would be a separate attack from the cavalry melee, too... you can't make a combined arms attack with the cavalry and infantry.