Between 18 and 21 is fine, but don't allow it to get any warmer. Is that the temperature of the room, or is that the temperature of the fermenter? I ask because an active fermentation like that can be much higher inside the fermenter.
Don't skim off the foam, just wait until it dies down in a few days. It sinks back into the beer, and then falls to the bottom and makes a compacted yeast cake on the bottom when it's done. Other things fall to the bottom too, hops debris, proteins in the beer, etc, to make the trub.
What do you mean by the foam is all over the hydrometer? Is the hydrometer inside the fermenter? Most people don't keep them inside the fermenter, since they would get covered with gunk and there is a risk of breaking. I'd get it out of there, and stick the airlock back on.
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