You can make anyone anything, really, but some characters are more suited for certain roles than others. You can just check their stats, and see who has the best magic power. That character would be a naturally apt mage.
Also, get the augment licenses first, they'll benefit you more at the beginning of the game than the equipments and skills will do.
Gambits aren't too hard.
The game checks them from the top and down. You first select a "condition", a condition is something that can happen in a fight, such as the HP of an ally dropping below 50%. Then you select an action to perform when this condition is met, and who to perform that action on.
Remember that the game prioritizes the gambits from the first down to the last, meaning if the first condition is met (for example that there is an enemy in range), then it will do that action without checking the second condition (for example that an ally's HP is below 50%). This is why you put what you consider the most important at the top.
By setting for example the "ally hp: < 50%"(means less than 50%) condition before a "foe: nearest visible" condition, the game AI will first check if there are any allies with less than 50% HP. If there are none, it will continue to check if there are any visible foes, and if there is, it will check which is closest, then perform the assigned action on them.