Yea, if your character is learning new skills or becoming more proficient in what they already have, my only requirement is that it be done in a way that makes sense. Like you said, they should be seen practicing it IC, not just randomly picking it up one day out of the blue. And it follows that magic characters would have an easier time with other magic-related skills, warrior-type characters with swords, etc. I won't require new skills to be adjacent to what you already have in the skill tree--again, just use common sense.