When it comes to a fusion gems powers, I feel that they wouldn't be aware of their additional powers unless they discover them by accident. So they would probably try to use their normal powers and then they would absorb a lot of energy causing them to overhead and melt.
@Loony And there is s training montage that ends in the formation of Fordite which Eve breaks out of because it disgusts her and is like, "That was completely crazy, you have failed at everything."