yes, that would be fine, you can of cousre use dry ice for one bench and run the other ones on air or water, but you would end up in the dry ice category, even if only one bench was made with dry ice.... otherwise it would be unfair.
and yes, adding fans is allowed, changing a waterblock or stuff like that is allowed.
the only thing that isnt would be going fron air to water, water to phase change or things like that :)