I'm no expert, but they look great to me! I like the way your characters stance portrays their personality. For instance, the last sprite looks like a cute little kid. The first 3 look pretty intimidating. 4th sprite looks laid back (eheheh).
The third one looks a little generic. I don't know any fighting style (accept for kickboxing) where you stand like this. This position makes one of your arms useless. Is he/she a kickboxer?