So, 6B is in testing now, there don't seem to be any issues yet.
Here's how the hulls now stack up:
You can see that there's quite a difference between them now. The larger ships can pack more equipment and take much more of a beating, and are more logistically efficient, assuming you have enough logistics points to field a fleet of them (don't worry, you will). On the other hand, they're proportionately more expensive to build.
Small is basically eliminated except for random events and the Drengin Super Dominator corvette. It's a shame, but there really wasn't a huge gap to fit it in.