What's the best way to clean out (can I use another chemical}your overflow tank after putting wrong coolant in and it thickened up quite a bit in my 2002 Jeep Liberty Sport?

Unbolt it,remove from vehicle and use bleach to clean ,then replace.

I am always really careful mixing bleach with anything (bleach and ammonia is deadly as an example); otherwise, do clean it.

Chassy is absolutely correct, mixing bleach (chlorine) and ammonia creates Chlorine gas which is dangerous to inhale (Chemistry 101), never mix them in the household or in cleaning the car. Use another cleaner.