The tagal were a species that Chris Metzen made up on the spot during the development of StarCraft and in the period since the game's release, no development has gone into fleshing out the tagal further. However, he is open to the possibility of tagal lore being developed. In terms of playability however, Metzen has expressed reluctance to make a fourth race playable, as it could diminish the dynamic between the terran, protoss and zerg factions.
It was stated in an interview by Blizzard employees that the zerg are universally feared, hated and hunted by all the sapient species of the Milky Way. The statement was quite general however, and it is unclear how much of it (if any) can be said to apply to the tagal.