If it was me I wouldn't risk it in that small of a tank, I don't think it's big enough for them to be able to have seperate territories. A fight is likely. Something like an occelaris would get the snot beat out of him and a maroon might kill your clarkii. Have you considered getting your clarkii a mate?
edit: for all those people out there who've done it successfully I think there are probably just as many who it didn't work out for.