If I were to go with a coolant flush every 15 to 20 thousand miles is this considered a little too excessive? It’s a ford fiesta mk 6 the service interval by the manufacturer is recommended at 100 thousand miles but I prefer to change it more often as my old Hyundai service interval was only 24 thousand miles.
If you're asking if this will hurt the car, the answer is absolutely not. Well, I guess there is a risk of wearing out the petcock sooner, but other than that, no. You will not void your warranty if you might be wondering if that was a concern.
The only real issue is your pocket book. If you are using the specified coolant from the manufacturer, there's no risk to your vehicle changing the coolant out at the given interval. In this case, by changing it sooner you are only wasting good coolant.