What happens if you want to add some coolant and don't have a mixture? Is that a bad thing, will there be any immediate results?

 

As long as you don't add too much coolant, it shouldn't be too harmful. Generally a mix is acceptable from 50/50 to 60 coolant / 40 water.

If you're adding a lot of coolant, then just go down to a store (auto preferably) and buy some distilled water and mix it. It's not worth the risk.