idling very HOT

hello peeps iv'e just changed my thermostat on my 1.8 cabby gti. now when i was given the thermostat i took it with happiness as it was a sunday and not many places re open!! so got home checked the receipt and found out it was a thermostat for a fiesta. im thinking opkay may be their kinda universal?? put it in statred the car starts to overheat!! now fan kicks in but does not stay on long enough to bring temp needle to middle again or anywere close. is this the thermostat causing this?