If you have solid plans to get a bigger tank soon, then I would go for it. Severums grow pretty slow; my green has only grown 3.5" in about 8 months, and he has outpaced the Rotkeil which has only grown 3" in the same amount of time. You'll have some time to get a bigger tank setup and going.
Severums can get aggressive towards each other even at a small size; the gold and the super red I had in my 180 gallon were fighting badly at less than 3" long. They liplocked and tore scales off of each other for no apparent reason and didn't stop for an entire day. I had to remove the gold severum. In a 55, you won't have enough room to keep more than 2 fish. Overcrowding could result in blood, and that's never fun. If the bigger tank is not a reality, then I would start with no more than a pair.
Having a spare 55 is always a nice thing too; you never know when you might need to pull a fish. I have a spare 55 gallon setup as a hospital/quarantine tank.