What fish would you recommend for a beginner who wants an aquarium?

Actually you never knew the true reason for your fish to be stressed after you installed the heater.

In my opinion, it can be one or more of the following reasons other than "they do not need the heater".

1. The heater raised the water temperature too quickly.
It is recommended not to change the water temperature faster than 3C per hour. Any quicker might cause fish to be stressed or even die. Since you have said your heater had only one setting. The temperature couldn't be adjusted. It is obvious that your heater have increased the water temperature all the way to its setting temperature without waiting for hours.

2. The heater might have raised the temperature too high.
Did you have a stand alone thermometer? Never trust the setting on the heater, since even the exactly same model of heater will produce slightly different water temperature under the same temperature setting. You have to use a separated thermometer to monitor the true water temperature. When the water temperature is way too high, the fish can be stressed.

3. Lack of oxygen in the water after the temperature had been raised.
When the water temperature had gone up, the oxygen in the water would be decreased accordingly. Since warmer water can hold less air. If you did not have an air pump to produce bubbles to cause enough water surface movement for gas exchange, the fish can be stressed for the lack of oxygen in the water.

By the way, I also would not count on the guppy themselves for population control. Yes, sometimes you might get lucky that they do not overpopulate your fish tank. There is always the chance for them to keep reproducing and overstock your aquarium to the point your filter can't handle the ammonia produced by them. The fish will die if the ammonia level couldn't be kept at 0ppm. Anyway, it is not a problem if you have a good plan for the fish fry. For those who have no idea what to do with the extra fish, it is the best to think ahead for population control.