I tend to buy via two principles, 1) in proportion to the in universe ship availability and 2) enough so that a game can be played reflecting those number. That being said, let's divide ships into the following categories:

Unique: Custom, prototype, or ships still in development.

TIE/Phantom*

Brights: 'Next Generation' ships not massed produced

TIE/Defender, E-wing

Advanced: Specialized ships

TIE/Bomber, TIE/Interceptor, A-wing, B-wing

Primary: Standard or mainline ships

TIE/Fighter, X-wing, Y-wing**

Outdated: Viable older ships

Z-95

*I know a lot of TIE/Phantoms were produced but, seeing as most we're destroyed pre-deployment, I'm considering it essentially a prototype ship. This is partially because other advanced models, like the TIE/Defender, have a greater presence while still being rare.

*I've placed the Y-wing in the Primary category because it was still being used like a mainline fighter, even though it was outdated.

My questions are 1) Is this agreeable? and 2) if you only get one unique ship, what ratio of the other ships do you get? Finally, 3) where would you put other well known EU starfighters -- Assault Gunboat, Missile boat, TIE/Avenger?

you should throw the Y wing in with the outdated viable ships.. it's from the clone wars

Yep I agree on the Y-Wing, it's is outdated but still commonly used. Also though the Tie Bomber was specialized in role, it was fairly mainstay in terms of numbers and use, and to be honest by the 3rd film I would say the Interceptor was going that way, it wasn't a specialty ship and wasn't particularly rare, it was being rolled out as a replacement to the Tie in a lot of places.