Yes you are right. For this we will have to do an effort. We will have to know exactly what difference of average both rims will make?
Theoretically, yes the mileage will be affected. but how much? It;s a tough question.
The rule is simple, if you want to save fuel, go for the 14 inch ones But no need to become afraid of suspension issue. Both rim sizes are being provided in stock by toyota.
Best of luck,