I don't think this is really all about making certain ships "viable", more expanding each particular ships options.
I think most of us are looking too far into this as a single ships boost versus adding an upgrade slot similar to titles but affects a broader portion of your squad than just 1 ship.
Think for example Youngster. His ability I believe only applies to TIE fighters, would that that in a sense limit the "diversity" of said build? He is strong and beneficial to certain aspects of your list but there are options for including non TIE fighters in your squad, the only limiting factors here are choosing a. The faction of your choice and b. What can be structured in X amount of points.
3X X-Wing's are only used for a reference here guys, and it's well established Rebels flew mixed squadrons for their mission purposes. I can honestly say there is no idea about "forced" inclusions of certain ships but rather an alternative option as to why you consider bringing this particular ship.
I could really get into adding a generic upgrade slot entitled; Squadron . It could bring in slot of so far under utilized Star Wars story.
Don't get me wrong, this could take a considerable amount of observation and thought to prevent certain aspects becoming skewed in our 100 pt death matchs, but what upgrade doesn't take significant consideration before it can be released in this era of X-wing.
Complex rules are certainly easy to make but simple, clean rules are truly challenging to manage into a streamlined game of this complexity.
And if you don't think 3 Rookie pilots and 3 Bandits are a 99 pt threat, ask the Palpmobile players that I ate for lunch. The only ship in this game you will hear Mr say is pure unpleasant and just poorly calculated is the Scyk.
I really want to meet the guy who play tested that ship on such a way that made it get hit with ****** over stick.
Long story short, adding a potential thematic element in the form of multi ship "Squadron" upgrade could make some very cool things happen. Don't let the possibility of bad overwhelm a potential Avenue of good healthy progress for this wonderful game.