The guppies are tropical fish, and provided you can keep the temperature of the pond at least in the 70's year round, they should do ok. I wouldn't let the temperature ever drop below 70, personally.
But, if you really want a beautiful small schooling fish, consider rainbow dace. Just a little minnow, but like all cyprinids, they'll eat mosquito larvae as happily as mosquito fish will, without being bullies about it.