guppies at 70F?

well...lets put it this way.
You live in hawaii, and lived there ALL your life, nice and warm, comfy and such.
then one day you are forced to live on the west coast, some 15-20 degrees cooler (relative to your temp rating, lol)anways, you have to live the same too, IE naked, lol
it would be uncomfortable, but you would survive, just wouldnt be happy

Guppies come from different envirements 70 would be ok as long as its a fairly constant temp. I have 2 tanks with no heaters temp stays 68 to 72. I have had no problems at all. As a matter of fact one of them is a female only tank and these babies are huge.

Yeah 70 is workable, but its not ideal...not for these fish. Can they survive, yes...but if you have an extra heater of no use in the future, definately use it for these fish. Guppies might be little pigs who breed like crazy but they are still tropical fish (most of them).

Ideal temp is 72 to 76.

I keep guppies in with my crayfish, somewhere between 70-73 usually.

Yea guppies are tough fish, shouldn't be a problem.