I'm currently right at the beginning stages of making my Tervigon, however as it stands now the only parts I'm going to need from the Fex kit are the legs and Crushing Claws, and only because I don't care for the spindly spider legged look of using Scytals for legs. Otherwise it's going to be entirely made from the Trygon.
My T-Fex on the other hand is made with only bitz from the Carnifex kit.
Both of these are including judicious use of Plasticard and Green Stuff however, and are not merely a kitbash.