"Very cool."
Yes, indeed.
"But Isolation doesn't work on just any phone case. Correct?"
It depends.. Some regular iPhone case can work. They just need to have the same special "property" with the case included in this purchase.
"You need to replace your iphone 6 case with the special, clear plastic, iphone 6 case that is part of the trick. Correct?"
Yes. But other recent iPhone models are supported too. I depends on your selected model during the purchase.
"How solid is the case, I currently have a monster phone case because the iphone 6 struck me as very breakable. Will this case hold up?"
It can hold up to the extent of a regular daily usage of a phone. It may just be not that protective as your monster phone case. Just treat the included case as a scratch and dent proofing case for the back of the iPhone.