When I was a lot younger I made the mistake of putting my guppies in with my goldfish and the guppies disappeared in a couple of days!!
If you value your guppies I would keep them alone.  There are different kinds of guppies and some are tropical in that they need a heater.  If you are not sure go back and visit the petstore where you got them from and there should be a sign on the tank whether they can be kept without a heater and at room temp or you may have to ask someone.  As far as food goes, I would stick to feeding guppies guppy food.