No, they can't. You might get away with it for awhile if they are juvies. Eventually the midas will get rid of the JD. Besides, a 55 is too small for a full grown midas. You would be better off with a 75g for the midas. The 55 would be perfect for the JD.
I have two midas and a pleco in a 125g now. The largest being 4" and he/she is a terror now. It has claimed the entire tank as its own.
that clown knife wont stand a chance. neither will the opther medium sized cichlids. As far as targets ive used larger Leporinus, and good sized Tinfoil barbs that worked ok. They definately took a beating though!
If the tank is under 180g, then nothing should be with the midas except maybe a pleco, or some robust target fish (and that would have to be at least 125g)....