If you're firmly decided already upon getting Legendary Event characters, then for the Emperor event I think you're more or less on the right track. Admiral Ackbar could be helpful to ward off the Shock debuff (and deliver a big heal to your squad if he dispels enough debuffs at once). But Biggs with Wedge, at least one taunting tank, and a third damager (Leia or Luke will be alright, either one of them) can be enough to just outpace the Emperor's damage. Fifth character can be yet another damager, a second tank, Ackbar, a Phoenix Squadron character... I don't think it makes a lot of difference as long as the core of your squad (Biggs, Wedge, Stormtrooper Han) is adequately geared & modded.
Personally I managed to beat the final tier of the Emperor event with a very mish-mash Rebel squad: Biggs, Ackbar, Leia, Scarif Rebel & K2SO all wearing gear level 8 and character levels around 65-70. So I wouldn't say the requirements are particularly steep or anything.
I reckon the R2-D2 event is significantly tougher. But if you gear up Emperor & Vader enough (as close to maximum as possible) and mod them fast enough, then one taunting tank (Royal Guard or Stormtrooper) and any two random damagers (whoever you find easiest/quickest to upgrade) at gear level 8 should easily suffice as support for EP & Vader.