I've always imagined the Alliance vessels as being like the imperial vessels, but grown up. The way both an Aston Martin and a Lamborghini are supercars, but the Aston is the grown up one. Imperial vessels look like they are marketed to very wealthy men, who haven't matured out of their teens (Lambo), the Alliance feels like a faction that's full of wealthy men (and women), that are truly grown ups(Aston Martin).
Exactly, the Alliance had a few hundred years to get their engineers some experience. The time for second-hand, asymmetrical, retro-looking ships seems to be well over.
They are the NEW power in the galaxy, they see themselves as the voice of PROGRESS, want to do things differently and they are ever so ready to decry the decadence of the old powers. So flying patched-together rustbuckets would actively contradict their ideology and message.
When i think Alliance i picture something advanced, well designed but not as shiny and comforting like Gutamaya. Clean lines but not as spartanic and aggressive as Core Dynamic's creations.
Something in between, like the Alliance itself is wedged between the old power blocks.
Utility would be more a matter of function than outright being visible.
I'm not saying that they should become the dedicated Alliance manufacturer but Zorgon Peterson ships do match these requirements surprisingly well.
They are very civilian, both in design and purpose, versatile and a better herald of the future than the dependable but not quite progressive Lakons.