That only applies to Freelancer and Mime, and you still need to master the classes to get those bonuses which is far more time consuming than you would think so it balances itself out.

You can give white magic to a Knight but you really would just get Cecil with his crappy white magic. The reason why is because the Job Classes raise certain stats and lower others, so giving magic to a knight makes him a more flexible fighter but his MP pool and magic stat will always be significantly lower than a proper mage because his class gives penalties to magic focused stats. In truth it's generally better to give skills and abilities from similar style classes to each other than trying to build a better Red Mage. Berserker is a powerful physical class in it's own right but giving it the Knight's Two-Handed ability makes it one of the best damage dealers in the game. Likewise Ninja's are awesome for having Dual-Wield but it's better in the hands of a Knight who has access to the strongest weapons in the game. That's how how I generally approach it but everyone has their own way of doing it so just experiment and see what works best for your play-style.

Yes, Freelancer and Mime have the potential to be the strongest classes in the game, but it requires tons of grinding to do so and frankly you can finish the game without resorting to it. In a way, it feels similar to FFIII where the Onion Knight is the best and worst job in the game depending on whether you bother hunting down the equipment and grinding to lv.99.