The base saves for a 12 HD magical beast are Fort +8, Ref +8, Will +4.
For stats, I'd recommend the following:
Str 2 Dex 19 Con 14 Int 5 Wis 14 Chr 4
The Dex 19 is derived from another agile swarm, the centipede swarm.
The other physical stats I based off the cranium rat swarm, which would seem to be about the closest sized magical beast swarm.
Feats, maybe: Alertness and Improved Initiative are good staples for feats, and fit these monsters well. Lightning Reflexes also makes sense. For the rest, maybe an Ability or Skill focus, perhaps Iron Will or Endurance?
It would work for the weapon melting portion of the ability (as you have it in Homebrews right now). If you don't think that is enough to warrant the feat, go ahead and swap it out for another.
I just figured it would help it melt those annoying fishing hooks that keep hitting it.