I realize it may be too late now, but I didn't know how to proceed. These hyssops have been in my yard for now two years, and the last year, they grew tremendously, drooping and becoming really messy. I really them for the butterflies and bees they attract, but there's no way we could allow them to regain their growth from where they were this last summer. So I decided to prune them this December (very mild December, here in the Northeast). So the question is, Did I overdid it, and cut too close to the beginning of the trunk? Should I cut even further?
blue hyssops pruning
You are well out of the growing season for Hissops and you should be able to prune like you did without consequences. The only time I've seen these hardy suckers die is from fungus type infections not over-pruning. I know its possible but it looks like you kept it to 1 ft so you should be fine. To keep from "shocking" the plant you should trim them after they bloom each time. It will thicken up the plant and you'll have shorter but nicer blooms.
By "trim after they bloom" do you mean cut it back to where it is now after they grow and have flowers and leaves in the springtime?
You can cut them to the ground in the fall-winter-spring without harming them. I do that here despite that the branches are woody and remain green.
Now, I presume that once there are new growths with leaves and flowers in the springtime, I shouldn't cut them, right?
Also, can I do the same to hydrangeas?
What kind of hydrangeas do you have? If you have Hydrangea arborescens, Hydrangea paniculata, or one of the reblooming cultivars of H. macrophylla then you can prune them now (although you don't have to). But if you have a non-reblooming cultivar of H. macrophylla (this is probably the most common type), they bloom on old wood so if you prune them now you won't get flowers next year. If you're not sure what kind you have, I would leave them alone until after they bloom. Hydrangeas also don't need to be pruned every year, so unless they've gotten overgrown I would leave them alone anyway. If all you want to do is trim off spent blooms that's fine, just don't do significant pruning.
Too late... Are the hydrageas gone, or they just going to skip 2013?
The plants should be fine (although I've never pruned a hydrangea all the way back to the ground so I'm not sure if that might cause a problem) but if they're the type that bloom on old wood then you won't have any blooms next year. In the future I'd definitely save your pruning for after blooming. Hydrangeas shouldn't need a severe pruning like that, if you need to prune at all I'd just do enough to take it down to the size you want.