Welcome to the forum!
Well, I have not heard anyone recommend using Drano or Liquid Plumr to flush a heater core. I would be very hesitant to try it. I suspect those may be a bit caustic if not completely flushed out. The heater core wall thickness is quite a bit thinner than that of home drains, and the types of materials that you're trying to flush out are different from what you will typically have in a home drain. Your home drain will typically be clogged due to organic materials such as minerals (calcium, lime) and/or hair. In a cooling system, you will typically have silicates, rust, plastic or metal (if your water pump vanes are disintegrating). I'm not sure how effective Drano or Liquid Plumr will be on these.
Probably the safest route to go is to back flush the heater core with water, being careful not to exceed the rated pressure for the cooling system. Distilled or de-ionized water would be preferred as well, but not too many household water supplies will provide that.
-Rod