I like that you are building on the 3e Swarm rule template.
I did a similar thing with my Mass Combat Unit rules (have you seen those Ron?), which I playtestested for a while. I even had a "swarm" Unit of juvenile carnivorous frogs at one point (I was running Greyhawk T-1: Hommlet & the Moathouse).
One thing I used in my mass-combat rules that worked very, very well was this: Swarms/Packs/Units only roll 1 Attack Test and then compare it to the Defense of all valid targets. I limited it to max 3 targets, with a bonus depending on how many Unit Members (or Hexes full of the Swarm in the new model) can attack the Target: +4/+2/+0 for bringing 3/2/1 Member against each target for Units, and the base +6/+3/+0 for Swarms/Packs, and then applying the Harried mod if the Attackers:Target ratio gets to 4:1 or greater. I then roll individual damage rolls on any target that takes a hit. This cut out many extraneous attack rolls as I was running enemies and helping the PCs run 2 Units of hireling armsmen.
I also tracked the Unit Size as a Ranked Stat, with each Wound having a possibility to kill a member. Each Wound scored would grant a bonus d6 damage, and it that d6 rolled a 5-6+ a Unit member died.
I think I'm going to reconsider this with the new Success Levels rule, with a Wound dropping a Swarm/Unit Rating by 1 and handwaving that Swarm members just die, but for Name-giver Units a Wound indicates a Unit member has fallen with a critical Wound (and may die without attention) whereas a Wound+5 indicates a straight up Death (good for immediate intimidation/Morale checks after the Troll decapitates a mook). Either Way, the Unit rating will be reduced, but I'll just have to track Casualties vs WIA (Wounded in Action, out of commission). This allows the Unit Rating to "heal" as the WIA guys may be brought back to full health.
I also tracked damage versus an abstracted Death rating, as you do with Swarms. Reducing the Unit Rating to 0 meant you've killed/incapped all of them, hitting the DR meant the Unit broke and ran with some surviving members escaping. Not all of this is applicable for Swarms, but I like how you are defining them based on how many Hexes they fill (with maybe a note on how many individual members fit in the hex).