The biggest problem with this system is most likely going to be the joints.
 
The simplest solution, which is what you might be trying to do, would be to fit the entire pony’s hind legs into the thighs and the entire front legs into the forearms. This way, you don’t have to worry about meshing the joints of the pony with the suit on the appendages. Depending on how the scale of a pony to the scale of a human, this system would have a margin of success. The main problem is that the joints on the torso would need to mesh up. While possible to do, this would probably mean custom bodies for specific ponies, not a one-size-fits-all suit, unless sizing could be changed through mechanical or magical means.
 
The more lazy solution would be just to define the inside of the suit as bigger than the outside. They do it for Sugar Cube Corner, why not a suit?

Almost resembles Button’s Mom.