You get a lot of ally points every day, so it'll come.
I don't really see your problem, you can farm all three (Storm, Magma, RG) as they're in different places.
Magma is in GW, so you can look for 3 purchases per day, or 15 shards per day. You need 124 (and have enough tokens for one additional purchase), so you're looking at 8 days to get him at 7*. Done deal.
Storm is in Cantina store as you say, and it's the best place to farm him. You calculated you need 7 additional shards, and those can easily be obtained by farming the three hard nodes (or even only the two 12-energy nodes). Done deal too.
You plan 253 16-energy Cantina battles, which represents, at the usual average drop rate of 30%, around 75 shards, for either RG or Snowtrooper. Snow can only be reliably farmed there, or through guild store and one hard node, and you need 106 shards for him. Farming his hard node for ten days would give an average of 9 shards, leaving you with 22 shards (on average) to get from the guild shop, or five purchases. In ten days, it could be done, but you're at the mercy of a streak of bad RNG. OTOH you only need 71 shards for RG, and he can also be farmed from two hard nodes (18 shards on average for 10 days), so you're looking at an average drop of (75+18) 93 shards - you'd have a nice security cushion in case of a bad RNG. And you'll be able to get back on Luke faster.
So, go Magma, Storm and RG, and you'll be able to try different combinations if needed.
Now, I don't know who are the three other Empire toons you have at 7* already. This could seriously change the advice (I suppose you don't have Shoretrooper at 7*, otherwise two more tanks are not needed).
BTW, moving this to Squad and Character strategy, where it belongs.