Most answers here are wrong... whatever you do with your coolants, the last you can do is cause corrosion. All the formulas will prevent corrosion.
The issue is with mixing coolants you can "milk" or "clog" them. That happens because the chemicals they use may interfere with one another. The problem is ALWAYS caused by the DEX ones. They added new stuff to prolong the life (they started to market one coolant flush every 100.000 miles!) and that damn stabilizers is what will clog your coolant lines if you mix it with posphate and some other kinds.
If you clog your lines, the least of your worries will be corrosion! you will melt your engine much earlier than that!
So, the only sane thing to do, as most manufacuters do not even list the contents of their coolants, is to stick with the same that's in there. And do not do that by color. Color means nothing. There are several ones that do not play along and are the same color.
For example, i just bought a gallon of the autozone or prestone "goes with everything" (though they still write in small letters on the back "does not go with everything, check your manual") which is orange, just to find out my car had some chrysler golden thing in there. I don't remember the names and details as it was 6mo ago, but after a small research on bob is the oil guy forums, i found out i would clog my system if i mixed them.
The bottom line is, if you mix you risk it. the chance nowadays are small. VERY small. but it's still there.
If you love the car, write down the coolant you are using (brand and chemical contents) and use only that type.
If you have no idea what's in there, flush, put new one, and WRITE down what you used. do not believe in color. Also, refuse to buy anything that does not have a list of 'ingredients'.
Now, for the performance, they are exactly the same thing. so, if you flush before, you are safe! you will NEVER damage your car if you use only one brand, whichever it is. regardless of what the manufacturer suggest. In fact, if you don't live in cold climate, you can use only distiled water and be done with it! (you just will have to top off every hour as most of it will boil and escape the pressure valve :) so don't do it ... but you will still get ZERO rust.
Now what to do if your level is low and you have to top off?
unless it's more than a gallon, and you never did it before, use distiled water. ALWAYS. only distiled water. It's the safest bet. your fluid now is 50/50 coolant and water. you are fine if you keep that mix from 30/50 to 70/50. so adding a gallon or so is totally fine!