I have been using a tranny cooler only for the past 3 years with no problems at all. My tranny rarely gets up to 150 and in the winter it runs at 120 or less which is technically too cold. I have not noticed any problems at all and I beat the tar out of it daily. I did not want to hook it up the the fiero tranny lines because they were much smaller diameter and I was worried about lack of flow. I think the best solution would be to plumb new lines (same size as donor car) up to the fiero radiator.
My 2 cents.
84 NB, 3800SC, E-85, VS Cam, 3.0 Pulley, 4T65E-HD, HP Tuners, AEM Wideband, Regal GS Gauges, S-10 Brake Booster. 1/4 mile -12.05 at 111.7mph