The 9th chevron address being a space gate was a highly unlikely possibility, since the only mass deployment of space gates known in Ancient history was in the Pegasus Galaxy. Milky Way gates are almost always found on a celestial body, even if it doesn't have an environment suitable for human life.But it was a bad decision because they had no idea where #9 would have dialed them to... for all Rush knew, it could have been a space gate, which not only would prevent them from continuing the Icarus project, but he would have guarenteed that everyone involved in the project would have died. They could have searched for another icarus planet or a different power source anytime, but they only had one shot to get out of there. He was just lucky that it went to a ship that (at the moment) had life support and power.
I see that you are also forgetting the fact that the Stargate was being overloaded with energy from the planet, which probably would've flung everyone out at high speed if they had dialed somewhere else in the Milky Way, judging by how the Icarus personnel were being tossed out the gate upon arrival to the Destiny. Then there would've been the risk of the gate connection switching to another planet, thereby separating them and increasing the likelihood that they would be screwed over upon exiting the gate if they happened to end up on a hostile or inhospitable planet.