Unless you plan to keep a fish that is temperature sensitive, I see no problem just using the well to keep your pond at a particular level. If you do something to aerate the water (preferably prior to introducing it to the pond), it sounds like all should be well. Even ignoring this would probably be ok if the volume of well water entering the pond wasn't particularly large relative to the overall size of the pond. Another potential problem from heavy use of well water is that well water can be oversaturated in gases (e.g., nitrogen), which can be bad for the fish. However, the limited volume of water you'll be adding should not be a problem, particularly if you aerate similar to what Cecil does with his buckets.
