That's a pretty shallow tank and a strong light, I would get a heat emitter instead of a lamp and run it on a rheostat (dimmer). You will need to keep an eye on temps and adjust as needed. What about moving them into tubs or a more suitable reptile cage and using heat mats? You'll need thermostats to control them but will keep temps much more stable and safe. 98 is much too warm. 76 is probably ok for the cool side but if your temps are fluctuating that much could cause issues. I wouldn't take petcos advice on reptile keeping at all.