Ideally GBR's should be kept with a sand substrate, it's essential in my opinion, they're earth eaters so it's only fair that they're given the correct environment, but if you have sand then everything is fine already! You could go for a shoaling Tetra, Cardinal Tetra's would be a good choice, considering that the Rams and Cardinal Tetra's both come from environments in which the water temperature is about 28-30oC, if kept in lower temperatures their immune system may become suppressed and they will be more susceptible to diseases such as Hole in the head and other fungal disease.