I think Hal would be pointless in this game, as some levels have built-in attacks from behind (negating his ability). Personally, I think Bubbles (Orange Bird) would be a fun addition.
In my opinion adding Bubbles, Hal and even Stella will be good for everyone. Their abilities can be useful for many levels, and Rovio will get green light to making next chapters harder (7-8 waves in level?). Also, it’s better to have 10 birds in flock than 7.
