a 29 gallon would suit 1 senegal, if you get a 55, you have MANY options.
look at a delhezi or a palmas polli. When i get my 55, i'm going to go with a delhezi or palmas, a rope fish and then a larger cichlid aswell.
But DO check out the delhezi and palmas polli, both are better lookers then the senegal.