well, true. 

Assassin droid and protocol droid is far easier to mix and remain consistent. They're usually roughly the same basic shape. astromechs and protocol droids not quite so close.

we got a bit of a crazy suggestion now...

how about instead of having the role list like this:
A female Jedi in distress
droid with slicing skills that talks
Sithly hero
etc.

have it on two lists... One with the types of characters you want (One jedi, one smuggler, one droid...) and a second list of skills you want spread out on your "crew" (Slicing, combat, floppy-eared comic relief, weapons' expert...)

That would give your players more wriggling room when they make their characters, but still achieve your goals for team makeup and team skills. Of course, once you get certain roles/skills filled, there'll still be some less for the rest to pick from.