Guppies I have kept over the years did much better in harder more alkaline waters, and a bit warmer water than many of the tetras who prefer softer more acidic water with a bit cooler temperatures.
Seems older guppies don't mind cooler temps but for young guppies the cooler temps appeared to slow growth considerably.
Tetras I have kept appeared to fair much better in soft to moderately hard water with temps not too much above 75 degrees F while guppies faired much better in hard water with temps around 80 to 82 degrees F. Just my two cents.
Pristella,bloodfin,and sivertip tetra's are more forgiving of water paramters than others. IME