Degus are fine with low humidity, it's high temperatures that can be dangerous. 78 is a little high for them but it should be OK if it doesn't get higher. You can give them a slab of marble or slate for them to lie in to cool down 
. They even sell one pre-cut for chinchillas called the "Chin Cooler" or something.
. They even sell one pre-cut for chinchillas called the "Chin Cooler" or something.
