Blowing out freeze out plug after start up

Water in the oil is not a good thing. Hopefully not for only a few minutes. I would suspect the wrong head gasket but a cracked head is a definite possibility. Coolant pressure is only around 15 pounds and unless the freeze plug was installed incorrectly it shouldn't blow out. It sounds like you're getting compression into the cooling system causing the plug blowout and the coolant in the oil. You should take it out and take it apart and look closely at the head gaskets and have the heads checked for cracks. Good luck and keep us posted.
toolman