What Frank Castle said....I think that'd work.
Only problem is getting all the fish at once. And what happens if you buy a species you really really want, after a few months you see that the particular one you have won't grow. Some fish are runts.
So now, you want to replace it but all your other fish are 6 inches. The only replacement you can find is 2-4 inches. Or, 2-3 of your fish catch a disease, and your eventual 240 is only half stocked now.
You'll end up having to add new fish, possibly smaller, to an existing community. Not saying it will happen, or that it's likely, but it happened to me.
That old thing about "best laid plans of mice and men".
Get what you want, try it. Everybody's mileage will vary.
you are absolutely right, even the best planning or intention may not work out. Always down to execution and intangibles.