I don't know how long it took to get different sizes but it takes a while. I've had mine for 5 years the first few started hitting the 7 inch mark around 3-4 years in. I got them at .5-.75 inches. Some are still in the 4-6 inch range. I've heard them growing a little faster and some slower. A colony is best, how many would depend on how big of a tank you have. I would say for 6 a 150+. I know people keep them in smaller tanks but from experience I would personally try for at least a 180gal set-up. I find my 150gal to be a bit small. Also make sure you have adequate filtration set-ups when they get larger - you will need it.