question regarding rhoms

I went into one of the lfs today to get a mate for my fighter, i enquired if they would be getting any different species of piranha in apart from red bellys. the manager said that they didnt have any on order but he had a rhom in the back which was around 4 inches in size and £45, my question is how long would this be able to stay in a 4 ft tank (if at all) and do you guys really think it would be worth getting rid of my oscar, sennagal birchir and rainbow cichlid to get it (i no that these fish would have to go anyway furthur down the line) thnx