In order for Marine VS Marine to work it would be difficult. I am not saying that its ever going to happen, but if its possible I would love to help.
It doesn't seem that difficult. All you have to do is de-compile the models , change the models to look like MK5 Heresy era armor in a 3D program and fix it in a way so Source won't reject it for no reason after recompiling.
Ok so that last part is the actual difficulty along with making Heresy Era Terminator armor. The rest can to done with a good coder. Both the Terminator and Fex have armor values for certain weapons according to Chrome himself.