It's always possible to wear different types of armour, but there's a catch. You cannot mix-n-match pieces of armour of different types. So for example, you cannot have Garment boots with Protector gloves and an Armor chest. They all must be the same type.
The only limit to what armour you can wear is how many sets of armour you can afford to buy.

But typically speaking, people choose a set and stick to it.
Armor type gives you the best Physical defense and the weakest Magical defense. It lowers your agility, making you slower, and increases your MP consumption, requiring you to use MP Pots more often because you're spending more when you cast spells.
Protector is a median set. It has average Physical and Magical defense. Your agility remains normal (I think) and there's still a slight increase in MP consumption.
Garment gives you the weakest Physical defense and the best Magical defense. It improves your agility by 20 and decreases your MP consumption.
As you're going to be using a spear, I'll assume your build will be mostly Int to take advantage of the spear's magic bias? In this case I would suggest either Garment or Protector, depending on whether your priority is with conserving your MP or HP.