it's certainly possible to win in these situations. it just isn't guaranteed. sometimes i prefer a game where i have a distinct disadvantage. the AI is good, but certainly not perfect. as long as you can get large hulls and the best defenses quickly, and you wage your wars slowly and cautiously, it's not too difficult to win from the corner. as nightboy pointed out, it's much easier to defend from the corner. i'm currently beating the drengin to a pulp. 1 large hull + 3 droid sentries + 2 nano rippers + 3 warp drive 5s = lots of dead monkeys. i just gotta pull the damaged ships out of front-line combat so they can repair. my low-industry planets are producing cheap fighters to serve as cannon fodder. my manufacturing capital is building medium class ships right now. once i establish a stronger beach head in the drengin's space, i'll need warships more evenly spread out. i may chose to fight this war in two parts.
as is often the case, the drengin are the largest military power in this game. a great deal of the military is soaked up defending thier vast empire. usually i play on loose clusters, but i wanted a less advantageous start so i picked tight clusters.
the drengin were completely alone in the largest single cluster, so not only do they have their typical military emphasis, but they've got an uncommonly powerful economic-industrial arm to back it up. but it's still only a series of AI algorithms. i declared war a few turns after the drengin attacked the torians, which was stupid. the torians are across the galaxy and had a moderate military about equal to mine (but i only own 7 planets). so i waited a few weeks while their offense fleets migrated across the galaxy before i struck. not only did i hit them while they were weaker than they should have been, but i also gained popularity for doing it, since it appears to the other AI that i was going to war on behalf of the torians. at least, i think they look at it that way. i've managed to wipe out a major chunk of their fleet and cut them down to my size on the graphs with only 9 capital ships. i've lost dozens of fighers for sure, but at about 170 to produce they're cheaper than my long-distance constructors. the drengin ships were all outfitted with shields, and they're only just coming out with armored ships now, which helped my initial advantage (i went around destroying as much as i could).
the drath are next, and will be a cake walk. but that's when i expect to start building animosity among the rest of the AI players. however having absorbed the drengin, i expect that i'll be unstoppable at that point. i was able to keep a technological edge early because a PQ 17 planet had a precursor library (+700% research bonus that occurs in the example mod provided with 1.2) AND 2 precusor labs. rebuilding the ruins of the drengin empire will be costly, and i'll probably fall behind technologically and need to retool my strategy.
but don't get me wrong. i've lost more than a few games playing this way. that's why i think it's fun.