I have a glass top too, and my temperature doesn't change that much. I keep it to about 78-79 and if it seems like it's getting a bit higher than that, i put a fan blowing across the surface and it chills down the tank to my normal temperature. I also leave part of the glass top open, so just part of the glass top (the one that sits below my light) remains unopened for the whole time. Sometimes when the light is off, i take the glass top out for a while too.
As far as damsels, i started with a 3 stripe damsel, and when i put in my clown, the damsel went after him. I tried to take her out, but i wasn't able to. I ended up taking out all the rock out, and then i was able to corner the damsel, but it was still hard, they are very quick!!