Is this shield designed by seeedstudio? I think that you can take the official library,change some part(e.g. startup step) and maybe you'll ok..you should also look about pin used by Serial connection.
if you'll use only general method like call a phone,send sms,connection internet, probably you won't have problem..if you want also use GPIO and similar things, you must extend the library.
If you want to implement your library, please DON'T use string and malloc,but only array of chars..