Sounds good, and that's more or less the way ships are really designed. Instead of a points pool, there's the budget and resources. Whether or not they're called classes, you're going to have a largest and a smallest. Once you have that, you can have something in the middle. With more fine tuning one ends up with a small-medium group, and a medium-large, for certain mission types or budgets. So I think your 5 groupings is spot on.
Regarding designers doing dumb things, history is littered with "classes" of only one or two ships. And, of course, we can't always blame the designers; often they're directed to do things that they do by other people/organizations.
Good luck with your game.