Small scorpions can DPS with their tails, but if you're playing the archetypal 'big fucking emperor scorpion' you're better off with the claws. If you full attack, toss that tail in, sure, but it's not worth it to take multiattack or any of that. You've got improved grab, build to improved grapple and power attack. Remember, power attacks don't lower your grapple checks, so you can dish out some insane damage, plus all that natural armor helps because you're going to be flat-footed a lot of the time. Don't bother taking ability focus with poison or any of that. It's only really useful if you're playing a bark scorpion for damage.
Of course the real weakness of any scorpion build is flyers, but that's why you've got your own flyers and spiders, especially. I love it-in my last campaign we managed to lure two dozen wasps into a trap set by the orb weaver webmaster. That'll teach those fuckers.
Though as far as DPS goes, if you're playing in an evil game, totally roll up an assassin bug. Their poisonous saliva is totally vicious, and the DC is high enough to get even tank-type creatures with high fort saves. Nothing starts a fight out well like a bite from an assassin bug the enemy thought was just a pile of dust. Of course, you'll have the problem of the hive-insects not trusting you, which can be a problem for surviving winter if the campaign is set in the higher or lower latitudes. You can either be honest about it, and take the Symbiosis feat, which works well if you're rolling with some Africanized bees because the alignment issues won't be a big deal, but if the local towns are regular bees then you better take Camouflage or Mimic. Stay away from ant hives, they're assholes regardless of what alignment you are, even if the campaign is set after 2006, settle for termites if you have too. Rooming with ants during the winter is just asking to be enslaved or eaten.