If you literally mean the life of the fish, none can live in the 55. 3-4 can go in the 75. If you are talking the literal life of a rhom, then a 75 wont be big enough. It will, however, last a very long time before it would be necessary to upgrade.
Now, advice you didnt ask for. You could put three reds in a 55 gal and they will be fine in there for quite a while. You could buy a rhom at almost any size, under 12" lets say, and you could keep it in the 75 for quite a long while. This advice is probably more realistic for you to follow unless you plan on keeping your piranhas for years and years, and want to be set up for life from the get go.