Well, with plenty of dry, organic material to absorb the poop you won't have to do it often.
My in-town setup had something like that and my solution was a kid with a rake to scoop the worst of it out and then just throw in more fresh pine straw.
If you post photos we might have suggestions. :)
My first thought is to enlist Boy Scouts, church youth group members, or kids from schools that require community service hours for graduation. :)
They should be fine.
60F is a much more comfortable temperature than 90F for chickens.
Since they're fully-feathered adults, as long as they have a choice between wet and dry and are getting wet because they CHOOSE to they'll be fine.