Amanda,
There are no immediate environmental repercussions connected to guppies, as far as I know, especially not when held in small ponds or even a bucket in a corner of your property.
Maybe you don't know guppies. They are small fresh water fish (cannot survive in salt or brakish water) and also live happily in gutters in i.e. Kenya, Tanzania, Brazil, Chile and even the mosquito plagued forests in Scandinavia where they have worked wonders - and that is a fact.
I have had hundreds, if not thousands of them in my time. These are not like the Talapia and other man-made "wonders".
They don't kill anything but mosquitos. They are actually being eaten by bigger things all the time.
Not a solution to all mosquito problems, of course; but just a suggestion and not a bad thing to have in a 5 gallon bucket underneath your house.