I guess it can't be ruled out as have read about them breeding on rare occasions, but as stated the fry are very suseptable to dieing.
Thing is to that some Sailfin's can appear 'guppy like' if they are interbred with other molly types to. I have seen a mail sal fin before that had a yellow main colour, and colour spots over the tail, but the guy who owned it simply said he has come from a tank with different molly breeds in it. On looking at the tank there where all manner of colour in there form the molly familly and no guppies.