Fairly self-explanatory. Each monster can use up to 4 skills, 3 innate and 1 gained by the Monster Skill support ability.
If a monster uses any skill (even a spell like Flare that normally requires MP), the monster will never spend MP. In fact, as long as the monster has more MP than what the spell requires, the monster will be able to use that spell. Also, monsters will always know their skills, regardless of JP cost. However, only enemy monsters will learn spells costing more than 0 JP if "learn with JP" is unflagged.
If a human uses a monster skillset, they will be able to learn the monster skill support ability.
Due to a bug in FFTPatcher, you cannot set any monster to use the last "small bomb" attack (clustered near bite and spark and self-destruct).