IMO a 55 gallon tank is too small for full time living.
I kept 4 similar sized Russians in a 20' x 6' outdoor pen and felt it was too small.
Then I kept a single male Russian in a 17' x 8' outdoor pen and felt it was too small.
I've had a 4' x 8' enclosure for a temporary set up and if I "had to"..that's the smallest I would go.
On the other hand, if the tort has a nice large garden to roam most days, the 55 gallon would do for a night time/occasional cold day indoor home.