Summons and Mercenaries are interesting in that +skills will actually boost their synergies, too. For instance, if you have a slvl 1 Firebolt and a slvl 1 Fireball, and you have +9 skills, the end result is a slvl 10 Fireball with 1 point synergy (the slvl 1 Firebolt). If a mercenary had that, then the end result would be a slvl 10 Fireball with 10 points synergy (the slvl 10 Firebolt after +skills). As a result, the +skills really boost a Merc's damage. If you can get your hands on a +3 Lightning or Charged Bolt Ormus, go for it, because it'll really crank the damage up.
I tend to prefer Cold Mercs to Lightning Mercs, since they deal slightly better damage, they have the Freezing Armor (which is handy for when they get swamped and saves on the cost of reviving), and because they freeze their enemies. In that case, be on the lookout for a +3 Ice Blast or +3 Glacial armor. Also, since the Fire Mage's spells (Inferno/Fireball) don't synergise each other, Fire Wolves deal pathetic damage and should be avoided at all costs.
The best Merc Rig is dual-spirits (be sure to make the Spirit Shield in an ethereal monarch or you'll never have enough strength to equip it), an Ormus with a bonus to a skill they use, and Griffons (for Lightning) or Nightwing (for Cold). If they're short on strength to equip the Spirit Shield (they won't have enough strength until they're in their 90s), stuff a PAmethyst into their armor or hat.
Also, yes, Mercs do benefit from FCR. I forget what their exact breakpoints are, but I'll look them up and post them sometime tomorrow for you. Another little thing you can do to speed up their firing rate is to get them some Faster Run/Walk (seriously). Since they spend so much time running around after they fire, a little bit of FR/W will get them where they're going quicker, so they can start firing again quicker. It really does speed up their killing speed. Stealth is a PHENOMINAL A3 Merc armor, because it's cheap and it's legitimately uber (FCR and FR/W).