Yeah, mixing almost never ends well, and is only accomplished successfully by experienced caretakers with massive tanks that are 100s or even 1000s of gallons. I would keep the milk frogs in their enclosure right now, convert the 90 gallon and use it for the 8 red eyes, and then later on give the milkys a nicer or larger enclosure if you so desire. I'm also curious about where you will keep 9 mossy frogs . Are you going to do 50% water and 50% land?