Right um, explanation time. Expies are basically something that shares a lot of the same characteristics as the source but at the same time different enough to be their own character. 

[img=http://static.tvtropes.org/pmwiki/pub/images/chars_everywhere_4729.jpg]

This is an example of expies: Char Aznable who inspired a whole slew of clones after him. Note that while all of them have masks and mystique, they are all uniquely different enough to not be the same person. You can use something close to canon mecha in terms of appearance and function, but outright using the form is kinda bad form for something considered an original mecha game. ^^.