The smallest Datnoid can still get 12" and since a standard 55 is 12" wide, you should really go with a bigger tank to begin with.
You could get away with keeping one (1) of the smaller species of dat in a 75 for life. It just wouldn't be the best thing to do.
You will really want to get a tank over hundred gallons to keep it for life with tank mates. And that may not even work if you are looking at a larger species of dat.
All that being said. Dats grow very slow so you could keep one in a 55 for a long time with tank mates, just not for life.