My son has bought some Guppies ??

Someone gave him a 1 gallon tank!

We keep our home at 18.5 degrees celcius
The water is at 55 degrees F.

Should they live ?????

18.5 degrees C is 65 degrees F. The tank water can't be 55 degrees F if the air is 10 degrees warmer. Regular guppies can live at 65 degrees F (although not ideal). Once you're down to 55 degrees F, I would expect them to start suffering and dying. Ideal regular guppy temperature is 70-75 degrees F. A 1 gallon tank is really too small for a heater (or much in the way of fish either). As long as you keep the house at 19 degrees C or warmer, they should survive, at least as far as temperature goes.

Guppies do like the tank warm but I have had a similar species the Platy in water as cold as 3 degrees and they thrived ! You do need a bigger tank and always have more girl guppies to boys, if you dont the females soon get stressed and die from the attention of too many male lovers.
Tropical fish prefer warm water but can in some cases tolerate cooler water.

I assume you mean 3 degrees Celcius as 3 degrees Fahrenheit would be well frozen. Even 3 degrees C sounds awfully cold for platies. I thought they died when below 50 degrees F or so.

Yeah so did I but I had no room in my tank so they lived in a large shallow barral in the garden At night it got very cold but during the day it warmed up but never higer than about 20 degrees C usually cooler.