Should I use an app prototype program like Pixate or InVision to make the app design?
Sketch is probably best since Fuse provides an Experimental importer for sketch: https://www.fusetools.com/docs/assets/sketch-import
You don’t have to use any specific tool, you can also do all your design prototyping inside of Fuse. 
It all just comes down to which tools and workflows you’re comfortable with.