Ok, take a deep breath!
Now that that is done, you are reacting pretty normally for any FTM, especially one of preemies. It will probably be ok if your pedi isn't worried about it. (DS had a couple of colds last year, and I panicked every time. Every time we got through it fine. We are on the tail end of his first this season, and I'm much more chill about it.) The big things to watch out for are fever and difficulty breathing. If she's breathing alright (for a baby with a cold), that is positive, and without a fever is positive, too.
Try not to beat yourself up about a few ailments. They will find their way into your home, unfortunately. Your H or you might bring them home from work or a store (or a visitor); they might get them from a trip to the pedi. Unfortunately, we cannot keep them in bubbles as much as we want to. It sounds to me like you are doing a really great job of keeping germs at bay, and that is all you can do: your best.
We have a Vicks humidifier, but I can't think of the name of it off the top of my head. We got it at BRU; it is white plastic with a blue vaporizer part. We also angled DS's mattress in his crib to help the mucous drain. It is tough having a sick preemie. T&Ps! GL!