Humidity won't care if it is hot or cold, just that it is set. If you can afford it, I'd invest in a cheap humidifier gauge. Petco has them for $2.99, often for reptile cages. That way you can see how dry it is in the room!
I'd definitely set up the cool mist one, in the meantime!
Be cautious about putting pots of hot water near keets, if they can at all get out of the cage.