Doppleganger. Is it possible?

I am trying to create a doppleganger character. This character has four abilities, each of which mimic a weak version of another class. The problem is everytime I make one of his shapeshifting skills and use it in the game, it crashes. I have tried using, storm crow form, bear form, burrow form, metamorphisis, and storm earth and fire. The latter being the only one that works. But since I cant use the skill as the base for all four abilities Im at an impasse. What is the best way to go about doing this?