You're not going to hurt anything with the 1/2 tsp/gal of Dyna Gro, but it's not really doing much to help at this time, either. Whenever you fertilize (with any amount), be sure to water to completion, so that water comes out the holes at the bottom of the pot, so you're flushing out some of the accumulated salts each time.
The young seedlings may skip dormancy but older plants are going to be harder to convince to keep going over the winter. I keep my Adenium outside year round (mild climate here) and it's never really completely dormant, but it definitely slows way down.
Since this is your first year with Adeniums, be sure to observe your plants as they go through their changes. The winter dormancy thing does not happen overnight, it's sort of a process, and you'll have to work out the watering based on what the plants tell you. When they are leafless they are not thirsty, and there's a real risk of rot if you overwater during winter, so err on the conservative side. And then start giving more water when you see them begin to leaf out in the spring.
I have never seen spider mites here and I think it's because they hate our high humidity, or at least that's what I've been told. They really thrive when the air is dry. Something for you to bear in mind if they should reappear. During active growth (when you're watering regularly and the plants are drinking deep) you might try misting them every so often to provide a disincentive to the mites.
