Hello,
So my house is regularly 20 degrees Celsius which is 68 degrees Fahrenheit. The thermostat is set at that and stays at that, is that warm enough for them? I've been told 21-24 degrees Celsius is best but is 20 too cold?

Yes, I would say it's on the chilly side. Temps between 72-78 degrees F is best.

If you have fleece pouches and fleece blankie squares in there they will snuggle up, but I would try to keep the room a bit warmer for them.

I would definitely either keep it warmer or put a warm lamp NEAR their cage, not right up next to it exactly but near enough that it generates warmth. This way they can go closer to the warmth if they are cold. I would always keep it above 72 for gliders, but that's me. They can die somewhere in the 60s, not sure exactly what.

This was posted from February when we were in the dead/cold of winter.

I am hoping that in Canada(where she is from), it is a balmy 75 degrees and approaching "Summer" in a few days!