I think they can definitely work as Kaiju!
The juvenile One Horn is a Bluefang, which are smaller than the Razorbacks or others. Due to the size and weight, you'll need the large base. Hex maps are probably out, but for 3D terrain it will definitely work.
Once you get to the mamas, bulls, and chieftains mechs get pretty small in comparison. It would be a cool set of rules to implement though!