I wrote a little more about this a while ago here, but really there isn't much to say. The fact is that left-handedness is never discussed as a problem for hoplites in any Greek source. There is no indication that there were left-handed hoplites. In fact, a lot of tactical problems hinged on the assumption that all hoplites were right-handed and therefore more effective at protecting their left side than their right. The most likely answer to your question is that left-handed people would be forced to fight right-handed regardless of aptitude, and would perhaps not do so well.