The Sheliak are indeed in the Alpha, perhaps Beta Quadrant, (To my knowledge) However both the Tamarians and Tama are in Delta and the Shelia (Sheliak) are in the Gamma. (not positive on Tama, but I remember seeing it in the Delta quad while playing the map).
Since, again to my knowledge, the exact placement of the Sheliak is unkown i'm guessing that is why they were placed in the Gamma Quadrant. (You'd have to ask Tnarg tho) I'm guessing he did this for balance issues. I'm fine with total unbalance to be honest and would be more than willing to play on a totally true to trek map
Even if I was the weakest of the 11 powers, not really sure who that is tho
Heck i'd even play as a minor if alll the race slots were filled, lol