1. I havent seen anything about elements vs monsters. in other words, im guessing that an obvious "fire" type monster would be less resistant to "ice" attacks. This is just based on some very limited trial, and just guessing from my other game exp.

question is: if this is the case, how do i know what a monster is weaker against which magic?

2. Until this point i have only been buying skills from the mage person. I noticed skills on the ranger and hunter people that would seem usefull to a mage, but im not sure if they only work well on that specific class or not.
question is: do the spell skills on ranger and hunter classes work just as well for mages?

2. You can buy any skill for any character. This is a classless game so you are not forced into any skill set or build.

Question #2 has already been answered.

As to question #1, experiment. But essentially you are correct. Poison-based creatures are going to be more resistant to poison-based attacks, and the same is going to be true for divine, shadow, fire, and ice. You will need to figure out which will work best for you.

Generally what I do, tho, is whatever my highest bonus is (which is given by the equipment I am wearing) is what I am generally using. Unless I am going up against a set of mobs that are resistant to it. (Which, for the most part, means that I am attacking with shadow-based spells, since that is where my highest bonus is at.) You can find your bonus on your character sheet.