I'd say you're pretty stocked up on fish for that 55, if anything maybe a firemouth or two. Jewels stay pretty small 4-5" full grown, so you want fish that stay in that same size range. Although my pair are in with fish that range from 5-9" and they more than hold their own.
I just put in a 5 inch convict with mine and he is 3 inches. As soon as the convict went in they went at it and the convict hasnt gone back since. He is the nastiest fish i have ever owned. Let me say again he is 3 inches and has no problem beating up my 11 inch red oscar! nasty fish i would probaly go with a south american in there but it would be dangerous with the other 5 jeweels