And it does make a difference as to what size the collector ports are. So I'll take a shot at your question.
Here's what I do after many trails and errors and experiments:
Bandsaw get connected to a serious 6.5 HP Rigid shop vac. it does make more dust
than you think.
Miter saw connected to a shop vac almost any one will do 3.5 Hp or greater the dust is generally propelled into the port by the rotating blade and the guard so it's easy to collect.
The tablesaw set up I have takes dust off the bottom, 4" port and off the blade when practical 2 1/2" overarm to a serious 6.5 HP Rigid shop vac. It sucks up far more dust off the blade than you would think!
Sander IS THE BIG ONE since it makes the most dust, assuming it's a belt or disc, not a ROS with a small port in which case a shop vac is what I use, but the adaptors are tricky in that case. I have a dual drum sander and it will create a dust storm in seconds.
The planer is a no brainer since it makes rather large chips rather than dust. Just hook the collector direct to the 4" flex hose off the collector.
If you can use a "separator" or cyclone before the Duct Collector that's even better. You said "bag" and we don't know if your bag is a 30, 5, 2.5, or 1.0 micron and that matters! The smaller the micron number the better. I personally would not use the 2" adaptor, but instead get a decent shop vac for the smaller ports.
A quick coupler can be rigged up using a 4" PVC coupler and the flared dust collector fittings from Woodcraft. Anything that's a slip fit will draw itself together when the machine is turned on and make switching between a snap. I hook the 4" to each machine directly and do not "Y" or use blast gates. But that's just me.