Here is an example of how the Rarity system would work and its use in combat.
Poggle the Lesser (lvl 1)
Geonosian Spy (lvl 2)
Geonosian Laborer (lvl 3) – light healer, focus on Geonosians. Very fast, but very weak
Geonosian Soldier (lvl 2)
Geonosian Warrior (lvl 4)
This would create a total of 12 Geonosians (1 Poggle; 2 spys; 3 Laborers; 2 Soldiers; 4 Warriors). Upgrading and promoting Poggle would only affect him. However, Upgrading and promoting the Soldiers would affect all soldiers a player has. One drawback is that using shards to create new Soldiers stacks with the shards used to promote them. So in essence you will be spending shards as though you were promoting them up to 8 stars, except 1 of the stars would be the extra Soldier (or 10 promotions for Warriors for the extra 3). When to promote verses when to get a new Soldier or Warrior (or ______) will be one of the many strategic decisions that all players, but especially new ones, will need to make.
The rarity level 4 characters should NOT be purchasable in the standard data packs (that would dilute their value far too much) (maybe not rarity level 3 either). Rather, they should be available in the normal light side and dark side battles. So players can obtain Geonosian Warriors by farming (but only by farming) the normal battles. This will level the playing field a little between players who don’t/can’t spend lots of money to get all the characters, and make it all around simpler for beginner players to obtain new (albeit somewhat weaker) characters early on. On the other hand, players who still want to pay money still can buy crystals and energy to farm as much as they want. Having a data pack specifically designated to level 3 rarity characters might work as well, but why do that when you can farm them and get credits and gear at the same time?
In the Galactic War, a player may vie to use Poggle with droids rather than with fellow Geonosians at first, while having combinations of the remaining 12 all together. However, if Poggle outlives the majority of droids, putting Poggle in with the other Geonosians (and maybe extra droids if they don't have characters like Lobot available) would be a perfectly viable strategy in order to continue getting as much use out of the Geonosian abilities as possible. Of course many strategies can emerge from these kind of combinations, this is just one strategy.