ur mugen is what u make it. u put the characters u want in it to have whatever cross over u want. just look in the chracter database section and u will find characters from different games. u can make ur mugen cvs, mvc/mvc2, mk, or a crossover of may different game characters and give it screenpacks that make it resemble whatever game u want it to be.
u can download a full game if u want, there is mvc eternity of heroes on this site. there are also other full mugen games but i can't recall any.
1 more thing
welcome to our world bro!