At the moment I only have guppies and have them at 25°. (77F)
What if there is a heatwave in the summer and temperatures rise to 30° (86F) - 35° (95F).
Have to add I'm from the Netherlands so it will only reach those temperatures in the early evening/night and then cool off again.
I assume this is the outside air temperature? When it gets that hot outside, what's the typical temperature inside the rooms where your tanks are? Tanks are slow to change temperature (the larger they are, the slower) so if it's only hot for a few hours, it may not be a problem at all.
+! for fans, cooler water changes, & frozen water bottles, etc. Likely all you'd need for a tank that size.
All this said, guppies are amazingly hardy fish, and would likely be just fine w/o any additional cooling. I. live in the desert, and one summer day the evaporative cooler stopped working for a few hours, and the inside temperature rose to 89˚F/32˚C. At the time I just had. guppies & otocinclus in my new tank, and they were all fine, if a little more animated than usual. I think the tank registered at 87˚F/31˚C. A top off with some cooler water and everyone was back to normal. The dog was a little more enervated than usual, but a couple of ice cubes and he was fine too.
Are you planning to keep other fish besides guppies? If so, just research their recommended temperature ranges. Many tropical fish are just fine at higher temps. And if you move up to 60g tanks, those will be much slower to heat up.