Hockey shin pads are significantly different to soccer or football shin pads as they need to be longer, wider, and have more padding than football shin pads to prevent injury if the event you collide with another player or against a goal post.
The ideal length of your shin pads for hockey is measured under the skate tongue, from top to middle of kneecap. If you wear them over the skate tongue, measure from one inch below the top of the tongue to the middle of your kneecap.
Hockey shin liners are worn underneath the shin guard to provide an under layer which cushions your leg while wicking away the moisture formed between your leg and the shin pad. They also help prevent the shin guard rubbing and chaffing your leg by providing this first layer of comfort.
Yes, all NHL players are required to wear shin guards.
Most players wear long socks under their shin guards either with or without long or short base layer pants. The longer the socks that you wear, the better protection against possible lacerations from skate blades.