Note that now you've turned the encounter pretty much on it's head, with the insect swarm being the major opponent, the rats being nothing more than a nuisance, and the zombie will likely go down in one round. I think I liked it better the other way around, i.e. the zombie being more prominent, the insect swarm being just a nuisance. The zombie should not be too far outmatched compared to the rats, because otherwise he won't survive long being eaten (well, perhaps "survive" isn't quite the right word here...)