Well, that is how I see it.
The most dangerous would be the machines, since they would actively seek out other races simply to destroy them. They would have all the technology from all the conquered races they have encountered in the past. They would devastate the galaxy. So lets hope there are none of them.
Advanced biological races would avoid the later races. It would be like a chimp coming into your house and asking if they could learn about your 'light switch'. Such a magical device that gives light when every you want it! Or to inspect your 'heating vent', that delivers warming vapors when its cold outside. Now I ask, how many of you understand how the heat vent works or how the light switch works, and would want to have to try and explain that in terms that the chimp would understand? Over and over and over again? Avoidance is easier.