You can use some detergent in it. The biggest thing is to get it filled back up with a top quality antifreeze ASAP. You may actually have to do this more than once if you have neglected the system and ran straight water or crappy super watered down antifreeze.
A lot of shops use Cascade dish powder to clean oil out of the system. It contains Dawn's grease cutter. For rust use a product called Thermocure or Evaporust specifically made to remove cooling system rust. Ford also has a product for that purpose. I don't remember what it's called but can look it up if you need it.