Yes, everything is very variable. I look at it this way when populating a reef. Fish only is similar.
#1 Is this fish reef safe? If not, forget it unless it is for a fish only system. Taking chances you might get that one dwarf angle that doesn't eat polyps is a big risk.
#2 What kind of environment is the fish use to? Does it have a territory of a few cubic feet, or miles of reef. I don't care how bad you want them, or what excuse you use, adding certain tangs to a too small system is disaster. Period, there are NO exceptions. No matter how many or few fish. While for example, a fire fish doesn't ever roam, even in nature, more than a few yards.
#3 Can I keep this fish fed? Will it accept a broad enough diet to lead a healthy and long life? Don't buy mandarins and butterfly fish unless you are ready to tackle feeding them, otherwise go burn your money in the street, at least you will get warm.
#4 Is this a territorial fish and do I have a setup that allows it to have its territory? Otherwise stress and fighting is in your future, then somebody dies.
#5 Does the fish look active, healthy and parasite free at the LFS
#6 How long has the fish been at the LFS
and is it gaining or loosing weight during its stay?
#7 Watch the fish eat at the LFS
. Don't take "sure, he eats like a pig."
#8 Populate the tank based on the adult fish's size. Don't con yourself that when that angel fish gets as big as a dinner plate in your 55 gallon, you're going out and buying that 300 gallon tank for him...it rarely happens,
#9 Get your information together before hand, or carry a smart phone. Don't trust the LFS
#10 Ask yourself, Am I impulse buying this fish and ignoring all I know I should do because I just HAVE to have him? This rarely works out.
I have seen systems teeming with fish that were healthy and have seen systems with just a few fish that weren't. It's all about the balance.
There are hundreds, maybe thousands of fish to pick from.
JIMO from a old LFS