Under the rules, you can't take Weapon Focus (mace), for the same reason you can't take Weapon Focus (sword) and gain the bonus with shortswords, longswords, bastard swords, and greatswords (and perhaps with rapiers and scimitars for good measure). Weapon Focus only applies to specific types of weapons -- that is, weapons with their own entry on the equipment table.
There's not a clear answer to your second question -- it depends on your character and your other priorities. How large is your character? If you're Small, you probably want Weapon Focus (light mace), unless you want to be restricted to using your mace with both hands. If you're Medium, there are good reasons for each -- you may want to use the light mace if you want to wield one mace in each hand, or if you want to master a light weapon that you can use under special circumstances (such as if you're in a grapple). But if these concerns aren't especially important to you, the heavy mace is a much better choice -- it does more damage.
