Yup, they can grow pretty fast in the first year - up to 3 or 4 inches. After that they slow down, maybe only growing 1" per year. Of course they can live a long time and get pretty big, maybe 12" in an aquaruim.
They are MUCH happier in a group. If you dont have room for several I suggest you take it back and get some smaller loaches. I have zebra loaches in one of my tanks, just as much fun as clown loaches, also eat snails but only grow to 4" max.