hmm at the end of that map i was allied with the thalan people. they even sent a troop transport an took the final drenign planet while my transport was on the way 
i picked the more diplomatic answers in the opening sequence and then once the map actually started i did some trades with the thalan (open border, some tech trades, sold them my first few durantium for some BC per turn etc.). also sent a freighter to their capital (and later with some higher trade tech some more freighters to other planets).
don't really know if my trading with them and doing open borders tipped the scale, but they were somewhat neutral to friendly the whole time and after i got the tech for alliances, they agreed to my alliance proposal.
so i guess it's not necessary to fight against them. you can actually end up with them as your allies.