[j.fro]

Back -flush, flush, back-flush. Then flush again. Combining the coolants is not the biggest issue here. When your water pump went, the plastic impeller had pieces of it (very small pieces) break off and go for a ride in your cooling system. Sooner or later something will get lodged in one of the tiny coolant passages in your cylinder heads. When that happens, you'll get an isolated hot-spot and the head will crack. The coolant temp gauge will never show it. Instead, the coolant tank will overflow in a mess of what looks like chocolate milk and you'll be headed for a very expensive repair. Trust me.