The Desert cacti and most of the other succulents are very tolerant of high temperatures and bright full sunshine as long as there is plenty of fresh air around them, but in a closed indoor environment or in a greenhouse /Polytunnel/ sun room/conservatory in the height of Summer when temperatures inside can easily reach well over the hundreds then the plants can ‘cook’ so although these plants love the heat and the sun ventilation is an absolute must, and try to avoid temperatures indoors above 90 F, this is not hard for me to do here in Ireland haha!
The Epiphytic / tropical rainforest cacti such as the Rhipsalis, Epiphyllum and Schlumbergera ( Christmas, Thanksgiving, Easter cactus ) prefer to be kept in a more shadier spot away from intense heat and sunshine, these plants are from tropical rainforests that are shadier and cooler and a lot more humid than the dry and intense heat of the arid regions where the desert cacti are from. Avoid exposing the Epiphytic / tropical rainforest cacti plants to very high temperatures during the high heat of Summer.
When it comes to overwintering these plants (check out my separate link to overwintering for more info) then the Desert Cacti and most of the other Succulents are happier to be kept at a minimum temperature of 7c around 45f as long as the soil medium is completely dry, and the plants are overwintered in a dry atmosphere away from excess damp and humidity. Some of the Mammillaria’s and Rebutia’s Agave’s can take very low temperatures during winter but I have no experience of keeping them below 4c for any long periods so I can not talk about this from my own experience. Melocacti and other warmth loving cacti such as Discocactus, and Uebelmannia need to be kept at a minimum temperature of 15c around 60f. The Epiphytic cacti also prefer to be kept at a minimum temperature no lower than 8-10c around 50f, lower than this and their stems are prone to scarring and spotting.
Seedlings need to be kept warmer during their first Winter and still lightly watered to prevent their delicate root hairs from drying out, because of this I would not recommend the temperature to drop any lower than 10c / 50F as seedlings are more sensitive to damp and rot than mature plants.
If you are overwintering your plants in a Greenhouse or Polytunnel an excellent greenhouse/polytunnel heater I recommend and use myself is the Biogreen Heater.