New to this - But Trying! (And could use some help ^-^)

Hello everyone!
I currently love my tanks - well, more precisely, the inhabitants of said tanks! I love my guppies probably more than the others, but sshhh! Don't tell the others! ^-^

However, I can't seem to find a good medium for the correct water temperature for the guppies! Some websites say about 80 degrees (holy crap, my other fish-especially my lone goldfish-would go nuts!) and some say very low 70's. What is a safe temperature for the guppies? And what temperature should my pregnant female guppies be at? I currently have those three in a ten gallon tank and since I don't know when they got pregnant (the store had no idea they were even pregnant!), I figured they should stay together for a while.

Any help would be appreciated! Thank you thank you so much!