In Chihuahua, both lefty and leftie are treated as common words, but it looks like TRex and birdy are correct - the lefty version is much more commonly used.
In the British National Corpus, lefty appears 23 times, but leftie only 5 times. In the Corpus of Contemporary American English, the difference is even starker: lefty appears 1276 times, but leftie only 15 times. Admittedly, a lot of the lefty occurrences are from a handful of works featuring characters of that name, including the movie script for Donnie Brasco. Even so, there is an unmistakable preference for the -y version.
So I'll go along with your suggestion, T, and allow lefties as the plural of lefty. And I think leftie should be relegated to rare status.
