For a couple years, if you get them at two inches or so, no problemo. Of course it would depend on the stocking of your tank and its inhabitants.
I've kept two in my 55 gallon with a discus, angel, and ghost knife (which will also outgrow the 55-- he's 11 inches now and still growing).
They are best in pairs or more. IME and that of many others, they are slow growers. My big guy has tripled in size but he's releasing hormones I think that is stunting the other. At some point I'm actually going to move the little one to my undestocked 29 gallon "grow-out" tank and try to put some length and girth on him.
Balas are kinda tricky in that some people think they are like redtail catfish, or even the massive and fast-growing tinfoil barb, which isn't the case. They are long-lived fish that get huge, near tankbusting size, but they are slow to moderate growers for the most part. Of course they would NOT be appropriate in a ten gallon tank any time longer than three weeks for Q. They need a lot of room to swim and 55 gallons give a pair of two-inchers more than adequate space.
Of course, as always, the ultimate size of the fish should be kept in mind. If you are planning on upgrading your tank with appropriate bala tankmates, then it wouldn't be a problem. Same if you are planning on trading, selling, or giving them away when they get larger. But if they will ultimately reside forever in the 55 gallon, they would outgrow it or get stunted and have a shortened lifespan, even though that would most likely take several years.
EDIT: Just realized this in the predatory fish section. Balas would not last long with predatory fish, even a big bala, I don't think.