Well, as long as you have enough attention for all of them i dont see the problem!
Oviously you need to consider vets bills ect as well, but as long as you're financially stable for another one than help the shelter out.
The most important thing when buying a cat is to remember its life span can be up to 20 years or more, you obviously know this but some people forget and its harder to rehome a 3 -4 yr old or older cat than it is a kitten!
I get so sad when people are leaving the country ect so need to rehome!

It would be nice to have all your cats around the same age group they'll get on very well, however you sound like a true cat lover and in a few years might get the kitten urge again! If you get broddy at the sight of kittens you might want to pace yourself