Can mother of groom and mother of bride wear long when bridesmaids wear short?

There is absolutely no reasons to not to! As long as your dress is not similar colour or style to the bridesmaid (and of course not white) then I think it should be fine. However, I think you have to in someway make it clear that you are the mother of the bride and not one of the bridesmaid - otherwise, a) the bridesmaid might not be happy thinking that they might not be valued and b) You played a big part in the Bride and the Groom's lives, you want to stand out and let people congratulate you on having a beautiful daughter